A KVM will probably use similar ports, rather than mixing and matching PS2 and USB. You could try a USB to PS2 Y-cable splitter from your Mac to the KVM, but check the KVM specs first because I know not all them support a Y-cable.
If you have your eye on a KVM that will support both PS2 and USB from the Mac, I'd like to know.
I wound up using a single monitor and seperate keyboards and mice for the PC and Mac. The PC keyboard controls the KVM toggle. Not a great solution. (I learned the Y-cable problem after I bought the KVM.)