This is really the primary value proposition of Free and Open Source Software. No matter what happens down the road, no one can suddenly jack up licensing fees. Even if one company decides to try to change the direction of a project (Oracle; OpenOffice), another group can fork it to keep it going (LibreOffice).
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).
Xen is a hypervisor. XenServer is the product name from Citrix which uses Xen underneath. Xen Cloud Platform is actually the name of opensource xenserver.
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.
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).