Can anyone share their experiences (for example, this was great! This failed miserably!) with using the [Hyper-V](http://en.wikipedia.org/wiki/Hyper-V), [ESXi](http://en.wikipedia.org/wiki/VMware_ESX#VMware_ESXi), and [XenServer](http://en.wikipedia.org/wiki/Xen#History) virtualization platforms? Cost? Management? features? Handling load and backups and recovery?

And also minimum server requirements?

I thought Xen was a free virtualization platform for Linux. Is there a Xen and a separate XenServer platform?

Opinions and observations would be appreciated for a test rollout for our organization.