I've got a 4-way USB powered KVM, you switch between the four systems with hot keys; <scroll lock><scroll lock><Port No. 1 to 4><Return> It looks like a cable mounted box with four tails.
Most of the time it works fine but on boot I can get a "No Keyboard connected" message and have to power cycle the selected system to pick it up.
I'm not sure where it picks the power up from, I assumed it was the VGA rather than the USB connection. N.B. only one USB connection to the system, keyboard & mouse have individual connectors to the KVM.
You can get passive KVMs, Belkin do them as do others, where you have to use a cable mounted switch to select the system (on a two port one anyway)
Hope this helps.