Vmware ESX server and GSX server used Transparent Virtualization technique.
Transparent virtualization allows operating systems, or particular components of the operating system, to run inside virtual machines without modification.
Paravirtualization requires the operating system to be modified before it can run inside the virtual machine. Depending on the part of the operating system being changed, the modification may be expected and supported by the operating system vendor (
e.g., new drivers) or not (e.g., changes to the kernel).
Xen uses Paravirtualization.