For Developers wanting to run XenServer on their development environment this could be helpful. This writeup is geared toward Mac Developers using Fusion.
Fusion is the recommended solution, as VirtualBox doesn't support nested virtualization.
The following configuration was used:
The following screenshot should help you through the steps to created the XenServer
Create a new Virtual Machine that will run XenServer, hit "Continue without Disc",
To do this you will need to create a special Virtual Machine that can be nested, select "Create a custom virtual machine" and Continue
From the Custom Virtual Machine Menu Select "VMware ESX" Operating System and Select "ESXi 5" and Continue
The number of cores and memory are set to the default:
You can press Finish to create the VM.
Attach the XenServer Installation ISO to the CD/DVD Drive
You can now startup the machine and it should boot from the ISO image and bring you to the XenServer installation: