The hypervisor is a thin operating system that sits between the hardware and the virtual machines.
XenDesktop is virtualization software, but it has to run on something. That something is hypervisor software.
Tribal Knowledge says that you can run XenDesktop on physical hardware without any hypervisor; however, there aren't many use cases for this.
For reference, in a Type-1 hypervisor deployment, XenDesktop runs on top of an operating system, which runs on top of a hypervisor, which runs on top of the server physical hardware.
The supported hypervisor operating systems are as follows: