Fully virtual: Complete emulation of all hardware devices. Although it requires special computer hardware, most operating systems can run in fully virtual mode because the VMM emulates all computer devices to fool the operating system into thinking it has exclusive access to an entire computer. This complete emulation of computer hardware demands more CPU resources from the VM Server. As a result, an operating system running in full virtualization mode runs slower.

Paravirtual: Selective emulation of hardware devices. An operating system that is optimized for the VMM is said to be VM-aware and can run in paravirtual mode. Paravirtual mode does not require complete emulation and therefore requires less management overhead. For example, VM-aware operating systems do not require an emulated graphics card, so the VM Server does not need to emulate video data. As a result, an operating system running in paravirtual mode demands fewer CPU resources and has better performance. It also requires no special computer hardware.