This post is pretty much a bunch of links that I have found that I am using to get up to speed with ZenServer. I've got a new virtualization project just around the corner and Vmware is no longer a good fit for us.
The link below is from Citrix.com and compares the various features in the different versions of XenServer.
And the link below explains XenServer licensing. There are three licensed versions of XenServer, and of course a free version. The specific version that I am looking at is Enterprise, which only lacks a few features available in Platinum. Street price for Enterprise is $2500 per server.
If you are interested in test driving a licensed version of XenServer, you can use the link below to register for a 45 day trial.
Last but not least, the link below is to Citrix's documentation portal.
Now just need to find the minimum and maximums docs and I should be good to go.