I use a D-Link 2port KVM, it uses kb keypress to switch pc and it requires the ps/2 ports to be plugged in to a powered port.

eg: the KVM PC2 cable is connected to xbox360 vga, but the kbd/mouse arent connected, the KVM wont switch to PC2, it assumes no pc is connected. I had to plug the kbd/mse to a powered PC to fool it.

i dont know how it goes on other brands, but better check first before u purchase it.