The Virtual Machine Monitor (VMM) is a thin layer that provides virtual x86 hardware to the overlying operating system. The virtual hardware is called a virtual machine and the operating system it runs is called the guest. It is through the monitor that the virtual machine leverages key technologies in the kernel such as memory management, scheduling, and the network and storage stacks.
Prior to the introduction of hardware support for virtualization, the VMM could only use software techniques for virtualizing x86 processors and providing virtual hardware. This software approach, binary translation (BT), was used for instruction set virtualization and shadow page tables for memory management unit virtualization . Today, both Intel and AMD provide hardware support for CPU virtualization with Intel VT-x and AMD-V, respectively. More recently they added support for memory management unit (MMU) virtualization with Intel EPT and AMD RVI.
The method of virtualization that the VMware VMM chooses for a particular guest on a certain platform is known as the monitor execution mode or simply monitor mode.
On modern x86 CPUs the VMM has an option of choosing from several possible monitor modes. However, not all modes provide similar performance. A lot depends on the available CPU features and the guest OS behavior. VMware ESX identifies the hardware platform and chooses a default monitor mode for a particular guest on that platform. This decision is made by the VMM based on the available CPU features on a platform and the guest behavior on that platform.