Most cheaper KVM switches do not have USB ports built into them or use USB to attach the Keyboard and Mouse. I have an 8 port Linksys and it uses PS2.
For one with USB look at a company called Raritan. They make some of the best KVMs out there but they do cost a little more. You get what you pay for.
I would spend a little more money on a KVM than $20 though. A crappy one is going to give you a crappy screen display and possibly cause other issues such as slow passthrough of mice and keyboard inputs. The Linksys I have isn't top of the line but it works flawlessy and the computer doesn't even know its there. It doesn't degrade the video signal at all. The only issue to look out for with Linksys and I'm sure others is that they use a proprietary cable.