I use a number of KVM units at work and the HP ones are by FAR the best.
I've only used them with HP servers ( 380, 385s and 580s ) but they're very near products. By far the best designed.
The ones I have come with a KVM dongle that you place on the back of each server, this turns the connection into a CAT5 cable and runs into the KVM unit, which can be placed on the back of the pull out monitor / keyboard / mouse. The whole thing only takes up 1U of space and it's a pretty sturdy bit of kit.
We have a number of Rittal ones at work, I can tell you now, unless they've changed in the last ~2 years when I got them, DO NOT BUY THESE. Nothing but trouble!!!
If you're using HP servers you could always consider using iLO though. We have started to adopt that now.