You can always download a free 60-day trial. I also heard that bloggers and certified experts can ask VMware to provide a free NFR (not-for-resale) license to be able to test the product in their home lab.
Veeam Community Manager
It would be much easier/better to run ESXi 5.x on your physical server and then, if you wish, run XenServer as a nested VM that runs on the physical ESXi host. Doing it the other way -- vice-versa -- is going to be more than a big challenge.
I run nested XenServer 5.6, nested Hyper-V 2012 R2 and nested KVM as VMs running on physical ESXi 5.x -- each of those hypervisors runs their own VMs.
Also, you're going to want to check to see if your Xeon CPUs are late-model enough to have EPT capability. Without EPT capabililty in your Xeons, you won't be able to do much with nesting hypervisors and performance will be abysmal.