So KM is keyboard-and-mouse sharing, while KVM is also sharing a monitor ("video"/"VGA" at the time).
My first try (some time ago). Had tried 2 devices, one in the 30€-, the other in the 100€-range. Both made bum-bum-bum. In the casings, there is a relay (or several such relays) which physically move(s) and realize(s) the connection(s), was only for kb, mouse and VGA then. Did work, but with lots of fuss, shifting to the other computer wasn't smooth but had to be made by press of a button each time (or was that only the cheap one, while the less cheap one's function was triggered by a key combination on the kb?), and then you heard the relais do their click-click-click, and while waiting for the shift to be made (2 seconds or so), you wondered how long the relais would work, and how long your computers and sole monitor would bear all this physical forth-and-back (electricity on-off-on-off...); I sent back both devices within the 14-day redemption period: When it's too loud, it cannot do any good for the material when the latter isn't made for all that (so this excludes construction machines, Ferrari motors and the like). It should be said that there also are some physical switches which then work by the network (starting (!) at 200€ or so), ie which do NOT then physically attach and de-attach electricity to your devices, and those are probably the way to go; let's call them "physical network switches".
The following software tools all expect your computers being connected to a network (LAN in my case, but they probably also work over a WLan?).
Microsoft "Garage" "Mouse without Borders" (free)
My next try, then, and again now. It's only KM (no monitor sharing), and it's free. No thrills, but it works, but on the 2 (or more?) computers, you are constantly invited to enter long, cryptic "security" codes of the other computer(s), and that quickly drove me nuts. There's an "explanation" somewhere how to avoid this constant nagging, but unfortunately I'm too dumb to get those hints working for me, so I preferred to dumb that unbearable tool instead.
ShareMouse (Bartels Media, now 30 / 50$ but incl. (!) VAT)
I had bought this tool at bits, for 20$ plus VAT, so Mr. Bartels got 10$ of mine for nothing, but I have to admit it was all my fault since he clearly invites customers to first trial his tool, then buy in case, and to not do the other way round, which I carelessly did, but that's the trick with sites like bitsdujour, many a times, software's just there for a day, without preadvice or without your having checked in time (both possibilities being common), and then you see it, you think it might be helpful, you buy it for cheap, and you store it for later use.
Of course, there's a hitch in Mr. Bartels' urging you to trial first, then buy - other developers rarely insist that much on your doing so, and for a reason. In fact, when you buy some tool which is declared compatible for Windows "n", and you intend to use it for that Windows version, you can reasonably expect it to run on your system. Not so with ShareMouse.
In fact, there should be lots of macro tools out there (or even almost all of them?) which use so-called kb hooks (whatever those may be), and it seems that ShareMouse uses such kb hooks, too, but whatever the real reason behind it may be, fact is, with my - quite current, especially for "professionals" - macro tool, ShareMouse does not work at all, or more precisely, whenever I trigger some macro on computer 1 or computer 2, I can never be sure upon WHICH of the two the macro (or parts of it) will work, so ShareMouse is worthless for me since it mixes up the 2 computers as far as macro input/execution goes (simple abc... input works well, though, doesn't mix up its current target computer).
It goes without saying that there is a high probability Mr. Bartels KNOWS about this incompatibility, so it would have be highly preferably, had he said instead, "ShareMouse possibly may have compatibility issues with some macro tool you have in use, so please trial first", but then, prospects would have been warned that even when ShareMouse worked well, currently, the day the also will install some macro program, they could run into deep trouble, and from a marketing point of view, it's understandable that a developer tries to avoid a situation where a prospect could get anxious about future problems when at least currently, all works well, so Mr. Bartels did not communicate this probable reason for his warning; the hitch in all this being though that such a KM is most notably used/needed/useful in "professional" environments/setups, and it's precisely there that users will also need/deploy a macro tool, too, if not today, then probably at a later point in time, so perhaps there will be in then for an unpleasant surprise even if their trial today goes smooth.
Also, this "kb hook" (or whatever it is) problem I only encountered with ShareMouse, not with any other KM/KVM tool, so it seems that "kb hooks" (or whatever ShareMouse's programming specificity is) are NOT needed to write such a tool; traditionally, programmers were put into 3 groups, mainframe developers, system developers, and application developers, and it's evident such a KM/KVM tool is system development, so possible interaction with other foreseeable system events (here: possible, concurrent use of a very common-with-professionals-ie-in-the-same-use-case-as-ShareMouse-itself macro tool) should be / should have been part of the development decisions, a "first trial!" urge (without the "necessary" explications) later on seeming a little bit lame as an alternative instead.
This being said, I only can speak for the previous version of ShareMouse here since the current version does not work for me: a mix-up of the current version on the Win10 comp, with the previous version running on the XP-comp just as little working as current version on both comps, it's just the previous version installed on both comps that'll work (and then and obviously very badly interacts with the macro tool, while it's futile to discuss if then the macro tool or ShareMouse is the culprit for that interaction, just let's say that IF it's kb hooks, that would be quite normal behavior for a macro tool while for a KM tool, it's obviously not - since the other KM tools don't come with that same problem).
Btw, incompatibilities in an XP-Win-7-and-further setup should not arise for a KM/KVM tool, in particular, since it's precisely in professional/corporate settings that the need for accessing (some) XP-driven and (many) computers driven by a more recent OS arises frequently, the bigger the corporation, the higher the probability for the existence of some XP computers needed for running some important, more or less made-to-measure software for special means.
ShareMouse is just a KM, not a KVM, but then, it seems to be unique (?) in working with mixed systems, incorporating Macs, so it's far from being all bad here - not owning a Mac, I cannot speak of its reliability here -, and ShareMouse has got a VERY pleasant feature I didn't encounter in any of its competitors: It allows for automatic "greying out" the currently-inactive screen(s?), and to a degree of your convenience; this way, you always see at a glance which one's the currently active monitor/pc (which is identical here since, as said, ShareMouse isn't a KVM), while on the other hand, the inactive monitors remains perfectly readable, so Mr. Bartels does it exactly the way his competitors SHOULD have done it, when in fact they all (?) realize this important function very badly... This being said, I'm waiting for this functionality being expanded to the indication of the currently-active monitor (ie the one where the mouse cursor is) even when several such monitors belong to the same computer (perhaps ShareMouse even does this, cannot say since I didn't try before de-installing): In fact, this latter functionality should be included in Windows itself.
And finally, ShareMouse - which I "trialled" extensively since I had paid for it - differentiates between the computers' own clipboard and clipboard sharing, ie whenever you try to insert the clipboard content of the/some other computer, you can't do it by control-v, but you must trigger a special key combination (default is shift-control-v here) which I, in my one-person set-up, found quite inconventient. This being said, perhaps ShareMouse also allows to simply (re-) set this key combination to control-v, so that ^v then works for both cases (I hadn't tried out this before de-installing the tool), and I have to say that behind this "quirk", there's another VERY good idea of Mr. Bartels: that in a 2-or-more-person set-up, this differentiation is really, really useful, just imagine what unwanted inserts would occur in the case of the usual ^v-for-all-inserts, irrespective of their repective origins.
Input Director (free)
While all of the software tools here allow for smooth clipboard sharing, Input Director is the only one of the bunch which does NOT also allow for smooth file copying made available by the KM/KVM, except of course for files, as in any network, being located in, and then copied to, dedicated shared-network-folders. The developer even tries to "sell" this lack as an advantage (security!) of his tool, but that's utter rubbish: In practice, every some minutes, you deeply miss this functionality which, as said, is present in all of the competition (? I don't remember Mouse without Borders so well, but I think the functionality is there, too) - ok, Input Director is free, but you'll quickly recognize it's a functionality that's simply "needed", highly expected, in such a tool.
But there's a point in what he mentions, and beyond what he says: Security considerations ARE worthwile, and in corporate environments, there should be some tracking device, for anything, anyway, and that would include anything such a KM/KVM tool would do, copies, moves, renames, and last but not least, changes of all kinds, incl. within (!) any files, not only changes made to their location.
The lack of file copying wasn't the only reason I quickly de-installed Input Director, though: in fact, it made my (shared) kb buggy. While my macros seemed to function normally, and regular kb input worked well, too, it wasn't but my PgUp/PgDn keys that run amok: Sometimes, they worked as expected, and then again, but again and again, pressing the PgDn key triggered the PgUp key instead, and vice versa, so that I finally only was able to use my mouse wheel to browse larger web pages (in several browsers) or longer text (in several editors, etc.). Of course, I tried with another kb, to no avail, and the same, "faulty" keyboards worked as expected again as soon as I deactivated Input Director, then finally de-installed it.
You know, sometimes developers get aware of some faults in their software which they cannot overcome, THEN decide to make it available for free, instead of selling it, which had been their initial intention though... if this applies to Input Director, too, I cannot say of course... Also, the visual indication of the currently-active screen/pc wasn't good (but I currently don't remember how it was done here).
Multiplicity (KM 20, KVM 40, 100$)
Multiplicity is from Stardocks (or from their branch-off Edge..., Ever.... Whatever), which obviously is the contrary of a one-man software software house, but which sells, besides lots of games, a lot of crap, their "Fences" being some quite prominent but particularly useless (and ugly) little tool for example. For non-big-corporations, Multiplicity comes in 3 flavors: 20$ for a KM for just 2 comps, and then 40$ / 100$ (always plus VAT) for up to 9 comps, but as a KVM, the 40$ versions handling the monitors of 2 of them though.
So while ShareMouse brings the unique (?) feature of Mac integration, Multiplicity comes with the unique (?) feature of screen sharing which undeniably is a world apart from simple KMs and which, if it works, obviously is incredibly useful, just imagine a very basic set-up, pc 1 with 2 screens, and one of these could be switched to/as the screen of pc 2 - in fact, I don't know if Multiplicity (in its 40-and-100$ versions) is able to do that since, while working with a mix-up of XP and later OSs, of all things monitor sharing is exluded here, while my remark from above applies here as well: It's especially in "professional" environments that users will probably have older and more recent OSs together in the same network, so excluding its USP, the monitor sharing, the "V" part, from its KVM capability then, should be a very bad decision of Multiplicity's - provided that technically, it should be possible to deviate some XP system's monitor-output to some screen of a pc driven by a more recent OS even if there's some hard work involved in order to do so.
So Stardocks obviously have got the manpower to do so, but as obviously, they have chosen to let their customers down with that, judging that their, Stardocks', effort would not be worthwile... for them, Stardocks again.
For my current set-up then, Multiplicity's theoretical full KVM functionality is worthless, but this deception notwithstanding, I considered buying their 40$-plus-VAT version anyway, for its smooth (but not very well-executed) KM functionality, shelving the (untrialled) "V" part in the tool for possible later use (their 30-days trial comprises the full 100$-functionality), but instead, I finally de-installed this tool, too, and here's why: First two minor "reasons", ie I was really unhappy about those points, but I would have bought anyway; then the reason that tipped the balance for me.
The indication of the currently-active screen is realized very badly, just compare with the fine way ShareMouse does it. You have 3 alternatives with Multiplicity: A quite / even not so tiny but ugly indicator quadrangle (with lots of unnecessary text, overlaying the text of your screen's content) for NON-active screen(s?): Depending on your screen's content, it's not so obvious the quadrangle is there, and/or your screen's content becomes unreadable there, and on your active screen, you're searching for the possible presence of that quadrangle before being able to decide, "this screen is the active one indeed!" - just awful! Then, you can get a colored frame around your active screen instead, and they even let you choose the frame's color, but it's so thin that even with a bright color, you never really know if it's your active screen or not (web pages, other pages using some color!), except for otherwise totally black-and-white screens (which are rare nowadays since even text editors use some colors now, e.g. for underlining, let alone for programming) - you never really know, I say, except, of course, for deliberately visually checking one of the four sides of that ugly and not even functional frame. Third alternative, greying out the inactive screen(s?), but without any setting for the degree (as Mr. Bartels does it so well in ShareMouse), but the inactive screen(s?) is/are? almost blackened, ie it's utterly difficult to "read", to discern anything over there if needed, and besides, it's ugly as hell - at the and of the day, with Multiplicity, you'll probably vote for the color frame around the active window and hope for the best, just checking your screens' edges again and again over the day - awful, I said.
Second consideration for not buying Multiplicity, but which I almost had decided to discard anyway: Their current version 3 is more than 3 years old (it's from April, 2014 if I'm not mistaken), and the (non-openly-communicated) update fees are (according to what some of their customers say) 80 p.c. of the full price: awful, again, but I wanted a functioning K(V)M, so...
What has been finally inacceptable for me, though, was the fact that I have reason to suspect Multiplicity to heavily interfere with my main system (the one where the paid - here: the trial-declared-as-"master" - version is installed): While it's understood that the K(V)M functionality of any such software tool isn't present anymore for any "slave" system while the "master" system wents to sleep, it's totally unacceptable that when you then try to wake up again your "master" system, it tells you that some "error" occurred, and that it will do the necessary things for you in order for shutting your system down and restart it... any data in open applications being lost, of course.
I don't have means to accuse Multiplicity to be responsible for this, I just can say I didn't see those error messages before installing Multiplicity, and they didn't occur after de-installing this tool; your system's mileage may vary of course. On top of this - and then I really was "done" with that tool, from one day to the next morning, I didn't have internet access from my main computer, whereas at the same time, web access from my (now) secondary computer (and from my internet telephone) was fine. I juggled around with the cables, to no avail of course - ditto for several browsers -, just as I had juggled around with the keyboards with regards to Input Director, days ago, and I juggled around, also without no effect whatsoever, with my firewall settings - does it really surprise you when I tell you that after de-installing Multiplicity (which implied a restart of the computer, too), I had web access without any problems, just like before? Oh, what another happenstance, right? (Oh, I forgot: in their "Object Desktop" (50$), their non-KVM, basic, 20$ version is included. And also, Multiplicity's screen dialogs appear as very "professional", so all its bad points then came quite as a surprise.)
Synergy (19$, with encryption 29$, last free version probably available somewhere)
As the others here, except for Multiplicity, it's just a KM, and I have to admit I didn't trial it anymore. After discovering Multiplicity, I had decided that when there are available both KMs and KVMs - well, in fact, just one single KVM, it seems -, a KVM would be so much more convenient (I didn't know yet that Multiplicity's "V" doesn't work with XP) - and it certainly will be if it works correctly -, I would want a KVM no, not a KM anymore.
Now, with Multiplicity having me disappointed so thoroughly, as a KM AND a KVM, it normally would be time to trial that other alternative, Synergy, but in fact, I don't really need a KM (or KVM) that much anymore, almost all of my "things" having been finally arrived at my new "main" computer, and at this point in time, I don't see that much need anymore for concurrently using my now "spare pc" with my main one; it's evident it'll come into play again whenever the main one lets me down, but it will not be concurrent use then, by design.
In other words, concurrent use of 2 comps - but with proper file "sharing", not like in Input Director" for one person is highly recommended in the transition phase of replacing one pc by a new one, and, of course, in corporate settings, for example where some person (who does not do this but on the side, here and again) also accesses the "server", for some technical-management means, but for one person really needing two computers running concurrently, I don't see any real-life use cases at this moment... and in fact, I only needed this when (re-) installing some system from ground up, both in the past and during these last weeks.
Which reminds me - of course - of my other thread, about trial conception, and over there, I would then have said, considering my current experience with KMs: "Don't do (otherwise uncrippled) 30-day trials for KM software, since most prospects will need your software for less than 30 consecutive days on the same system anyway, so by such a trial, you'd provide it to them for free."
And now have a look at Mr. Bartel's trial and compare it to the others' ones, and yes, they all can learn from his. Or in other words: Shelve current trials (well, except Bartels' one) - if you can live with the quirks those programs unfortunately seem to present, probably not just within my set-up (- and probably Synergy isn't without fault either?) -, in case you'll ever need them for some time, and the respective developers will have changed crucial trial characteristics in the meantime. EDIT: Perhaps ShareMouse's free version is a real free version now, not a trial anymore? I've known it stopping working after some minutes every time, then closing the tool and invoking it anew was necessary if I remember well. In any case, continuous use was impossible.
And let's remember Mouse without Borders (which is free, not a trial): Perhaps you'll overcome my problem with its endlessly asking for inscrutable codes, and it'll make an acceptable KM for you, whilst coming without any thrills indeed.