For Microsoft Virtual Server, the most promising course that I could find was the official Microsoft E-Learning course: Using Microsoft Virtual Server 2005. I suspect that this might be the case because the upcoming hypervisor in Windows Longhorn Server could render the Virtual Server product obsolete.
For Xensource, they offer their partner program. You can get training if you are a Xensource Solution Provider (XSP) or Xensource Hosting Partner(XHP). I believe that the training is included in the partner program.
This was first published in January 2007