A hypervisor or virtual machine monitor (VMM) is a piece of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor is running one or more virtual machines is defined as a host machine. Each virtual machine is called a guest machine. The hypervisor presents the guest operating systems with a virtual operating platform and manages the execution of the guest operating systems. Multiple instances of a variety of operating systems may share the virtualized hardware resources.
A hypervisor, also called a virtual machine manager, is a program that allows multiple operating systems to share a single hardware host. Each operating system appears to have the host's processor, memory, and other resources all to itself. However, the hypervisor is actually controlling the host processor and resources, allocating what is needed to each operating system in turn and making sure that the guest operating systems (called virtual machines) cannot disrupt each other.
virtual operating platform
Computer hardware virtualization is the virtualization of computers or operating systems. It hides the physical characteristics of a computing platform from users, instead showing another abstract computing platform
A virtual machine (VM) is a software-based emulation of a computer. Virtual machines operate based on the computer architecture and functions of a real or hypothetical computer.
Reasons for virtualization
In the case of server consolidation, many small physical servers are replaced by one larger physical server to increase the utilization of costly hardware resources such as CPU. Although hardware is consolidated, typically OSs are not. Instead, each OS running on a physical server becomes converted to a distinct OS running inside a virtual machine. The large server can "host" many such "guest" virtual machines. This is known as Physical-to-Virtual (P2V) transformation.
Consolidating servers can also have the added benefit of reducing energy consumption. A typical server runs at 425 W and VMware estimates an average server consolidation ratio of 10:1.
A virtual machine can be more easily controlled and inspected from outside than a physical one, and its configuration is more flexible. This is very useful in kernel development and for teaching operating system courses.
A new virtual machine can be provisioned as needed without the need for an up-front hardware purchase.
A virtual machine can easily be relocated from one physical machine to another as needed. For example, a salesperson going to a customer can copy a virtual machine with the demonstration software to his laptop, without the need to transport the physical computer. Likewise, an error inside a virtual machine does not harm the host system, so there is no risk of breaking down the OS on the laptop.
Because of the easy relocation, virtual machines can be used in disaster recovery scenarios.