Virtual Machine Monitor

For virtualization is a virtual machine monitor (also known as Hypervisor). It can be defined as “a layer of software that emulates hardware of a complete computer system” [1]. The abstraction created by VMM is called a virtual machine. There are three types of VMM according to their structure:-
Type 1: These VMM (Figure 1) are directly installed on the hardware. They are also known as stand alone VMM.Examples-IBM 370, VMware ESX server, Xen.
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Type 2: They (Figure 2) are installed on the host operating system. The operating system running on physical machine is called host OS and one running on virtual machine is called Guest OS.Examples- UML (User Mode Linux).
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Type 3: This (Figure 3) type is a cross combination of Type 1 and Type 2 VMM. They mostly operate on physical hardware but use services of host OS to perform I/O activities.Examples- Microsoft Virtual PC, VMware workstation. 
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Virtual machine monitor runs in Supervisor mode and controls access to resources shared by virtual machines. It schedules virtual machines in manner similar to OS schedules processes. Any non-privileged instruction is executed by hardware directly while for privileged instructions interrupt is generated to allow VMM to handle the execution.
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