Alternatives to VMware like KVM aren't just a better version of VMware that happens to be free, but it's fundamentally better because it is free (as in speech, of course).
I know its being a stickler for the tiny facts, but when you are a XenServer admin find the differences rather quickly.
You wouldn't use just Xen as a hypervisor, in most cases. Amazon uses Xen plus their own stuff on top.
To make it more complicated, there's also XenClient, which also builds on Xen. It's also mostly open source.