The hypervisor supports hardware-level virtualization on bare metal devices like CPU, memory, disk and network interfaces.

The hypervisor software sits directly between the physical hardware and its OS. This virtualization layer is referred to as either the VMM or the hypervisor.

The hypervisor provides hyper calls for the guest OSes and applications.

A hypervisor must be able to convert physical devices into virtual resources dedicated for the deployed VM to use. Depending on the functionality, a hypervisor can be a micro-kernel architecture or a monolithic hypervisor architecture.

A micro-kernel hypervisor includes only the basic and unchanging functions. The device drivers and other changeable components are outside the hypervisor. Eg. The Microsoft Hyper-V.

A monolithic hypervisor implements all the aforementioned functions, including those of the device drivers.

Therefore, the size of the hypervisor code of a micro-kernel hypervisor is smaller than that of a monolithic hypervisor.

Ex.VMware ESX for server virtualization.

There are two types of hypervisors: Type 1 and Type 2. Type 1 hypervisors run directly on the system hardware. They are often referred to as a "native" or "bare metal" or "embedded" hypervisorsin vendor literature. Type 2 hypervisors run on a host operating system.

Type 1 hypervisor: This is also known as Bare Metal or Embedded or Native Hypervisor.

It works directly on the hardware of the host and can monitor operating systems that run above the hypervisor.

It is completely independent from the Operating System. The hypervisor is small as its main task is sharing and managing hardware resources between different operating systems. A major advantage is that any problems in one virtual machine or guest operating system do not affect the other guest operating systems running on the hypervisor. Examples: VMware ESXi Server Microsoft Hyper-V Citrix/Xen Server

Type 2 Hypervisor This is also known as Hosted Hypervisor.

In this case, the hypervisor is installed on an operating system and then supports other operating systems above it.

It is completely dependent on host Operating System for its operations

While having a base operating system allows better specification of policies, any problems in the base operating system a ffects the entire system as well even if the hypervisor running above the base OS is secure.

Examples: VMware Workstation Microsoft Virtual PC Oracle Virtual Box