Before we get started, you will need to understand what a hypervisor is. A hypervisor is a thin operating system that hosts multiple instances of disparate operating systems. It can also be defined as software that can create and run virtual machines. The hypervisor software runs on server hardware that is enabled for virtualization. Once this is installed, you can install several instances of different operating systems onto the hypervisor. The hypervisor was game-changing, because instead of running one operating system per server, you could now run X number of operating systems on one server, saving space and money.
There are several vendors that make hypervisors, such as Citrix XenServer, VMware ESXi, Microsoft Hyper-V, and the Linux open source Kernel Virtual Machine (KVM). There are two types of hypervisors:
As you can imagine, the Type 1 hypervisors have been touted to have better performance as they interact directly with the server hardware resources.
Citrix XenServer and VMware ESXi are Type 1 hypervisors. Microsoft Hyper-V is presumably a Type 1 hypervisor. There has been debate over whether Hyper-V is a Type 1 or Type 2 hypervisor mainly because you first install the Windows server operating system and then turn on the Hyper-V role, giving the perception that Hyper-V is running on top of or with the help of the Windows Server operating system. Obviously, XenDesktop runs on Citrix XenServer, but it can also run on VMware ESXi and Microsoft Hyper-V.
Note
At the time of this writing, XenDesktop does not run on KVM. You could probably make it work. Citrix does not officially support KVM. Tribal Knowledge says you could run XenDesktop on KVM, but you would not be able to use MCS or PVS automation for creating VMs.
The following diagram gives you a visual idea of the differences between the types of hypervisors compared to traditional servers. It also shows how the interaction between these components contends for hardware resources and ultimately affects performance and sizing of hardware resources:
