I've played around with VMware for years. Not on servers, but on my desktop. I was really familiar with the way it worked when I found VirtualBox, which became my desktop emulator of choice. They both worked in very similar ways, where they would completely emulate an entire machine. Then Xen came out, and I was pretty lost.
I've never taken the time to do the hard work in acquiring a mastery of Xen. I didn't really see the need for it, and I didn't understand it well. I still don't understand it all, but yesterday Joe Topjian posted a link on Adminspotting to an outstanding Introduction to Xen on Debian. A very small bit of the instructions are Debian specific, but it functions well as a general introduction to Xen. There are some questions unanswered, but it seems like a great starting point.
If you are similarly unfamiliar but curious, definitely check it out.