There’s been plenty of discussion about the steps necessary to run VMware ESX inside VMware Workstation. But most of us haven’t yet invested the time in our hypervsior triumvirate’s least-deployed third: Citrix XenServer.
Getting the XenServer software starts by clicking on over to Citrix’s website, specifically here. In this location, you’ll be given the option to download either XenServer's free edition or a 45-day evaluation of XenServer Platinum Edition. Either will work for a test drive.
Once you’ve gotten the software, getting it running (perhaps, on your dedicated virtualization learning laptop?) requires the same VM-in-a-VM steps that you already know for nesting ESX inside VMware Workstation with one or two minor modifications.
Most important is the recognition that you’ll be running XenServer not as an ESX OS, but instead as an “Other Linux 2.6.x kernel 64-bit” OS. An LSI Logic SCSI controller is also important. But that’s just about it. Give it a processor or two and at least 1G of RAM. Connecting the resulting VM to your XenServer media should complete the install without issue.
If you want the entire step-by-step, I’ve found a nice write-up here.