A CPU architecture is virtualizable if it can support the basic VMM technique of direct execution; executing the virtual machine on the physical machine, while allowing the VMM keep ultimate control of the CPU [3].
In traditional systems, operating system owns the hardware and runs in privileged mode while applications have lower privilege. In virtualization, VMM requires the highest privilege level in order to establish isolation between virtual machines as well as improving its own performance.
Traditional VMM relies on de-privileging, running the privileged guest operating system at user level.