Short for Keyboard/Video/Mouse. They are standard equipment for server cabinets and IT desktop labs. Also great for quickly sharing your big monitor and your keyboard with your laptop while at home. The big brands are Raritan and Avocent (formerly Cybex). I definitely prefer Raritan. Compaq/HP makes some, but I don't particularly like them.
There are two basic approaches. One connects your PCs directly to the KVM via cables - this is what you want. The other ("remote KVM") connects a small device to your PC, then runs over cat-5 cable to a KVM in another part of the room. The higher end ones I use can be controlled remotely through a web browser, or up to 500' to 1,000' feet away over regular cat-5 cable.
They are sized by ports. 4 ports is a good size for home or SOHO use. 2 port is cheaper, but sooner or later you'll wish you had a 3rd port (like when moving to a new PC). Get the 4 port if you can afford it. 8 ports is standard for a server cabinet. 32 ports is good for remote KVM.
Belkin makes a decent line for home use, called the OmniView series. The 4 port model F1DB104P runs $70 at my professional discount. (I'm not sure what retail runs.) Raritan has an inexpensive SwitchMan line which is much better quality SOHO, but costs more ($95 for 2 port with 2 cables part # SW2-2C, $130 for 4 port with 2 cables # SW4-2C). Avocent also makes a SOHO line, the SwitchView series. CDW carries many different models.
http://www.cdw.com (Pro units run $400 to $8,000 - not what you need!)
Make sure the cables come with it or you include them in your price comparisons. The cables run $20 to $50 per set, depending on length and quality. 6' should be a good length for you. SOHO units come with 2 sets of cables and you buy more as you need them. Pro units come with none, as we often have them in stock. There are different types of cables, so make sure you get the right ones for your unit.
One end of the the KVM cables plug into the regular keyboard, mouse (these are called PS/2 plugs) and monitor jacks on your PCs. The other plug into the KVM. The cables you use now will plug into the output of the KVM.
There are also USB units out now. Priced about 15% higher. Strictly for home PC use.
Pro units do things like send "keep alive signals" to your mouse ports. On the cheap home units, you will occasionally have to reboot to get your mouse to start working again.
Also, check the video resolution. Make sure the KVM can handle at least 1024x768 and 72Hz, or you'll get lousy video.
Tip: Learn the keyboard shortcuts. Each manufacture has a hot key that lets you switch right from your keyboard (such as Ctrl+Ctrl or ScrlLk+ScrlLk), instead of messing with the buttons on the KVM. This makes them much easier to use and lets you keep the unit off your desk, since then you don't have to touch it to switch. Most units also have a scan function which will automatically switch between screens every few seconds for monitoring progress.
Tip: Most brands let you name the channels. This lets you see them in plain English ("Dell PC", "Laptop") instead of just "A" and "B". One time set up and pretty simple.
Tip: All these manufacturers have good tech support on their websites.
Tip: If your KVM is giving you problems, cycle the power (turn it off and back on). That cures most KVM ills.
Tip: KVMs do require electric power. If you use a UPS, connect the KVM to the UPS or you won't be able to control your PC if the power goes out. They draw next to nothing, so your battery will last just as long.
Probably more than you wanted to know. Good luck.