Virtualization Terminology Index

Virtualization

Virtualization is a methodology to allow multiple execution environments (known as virtual machines) to run independently and concurrently on the same hardware through the use of use of concepts such as partitioning, simulation and others.The virtualization software layer provides a logical rather than a physical view of the hardware resources. No changes are required to the guest operating system running on the virtual machines.

Paravirtualization

Paravirtualization is a virtualization technique that presents the abstraction of virtual machines with a software interface that is similar but not identical to that of the underlying hardware. In this case operating systems running on the virtual machines will need to be slightly modified.

Virtual Machine Monitor (VMM)

A virtual machine monitor is the layer of software that allows multiple operating systems in complete isolation from each other, which can include different types of operating systems (Linux, Windows, Unix,...) or multiple instances of the same operating system, to share a single hardware platform or multiple independent hardware platforms. Each instance of the operating system runs its own applications as if it were the only OS in the computer and has access to the underlying hardware through the VMM.

Hypervisor

Hypervisor is synonymous with Virtual machine monitor. IBM currently has the trademark for the term "hypervisor".

Virtual Machine (VM)

A virtual machine is one instance of an operating system running on a "virtualized" computer. The virtualization is accomplished by a layer of software called a "virtual machine monitor" (VMM). The virtual machine has access to the underlying hardware through the VMM.

Virtualization Layer

The software layer provided by a Virtual Machine Manager that supports one or more Virtual Machines.

Physical Layer

This is the physical machine supporting the VMM, it constitutes all the hardware resources that are available to it.

Type-I VMM

Type-I virtual machine monitors run directly on the bare hardware. Those VMM typically have an embedded kernel that can be from any type of OS (Linux, Windows, Unix,...).This VMM offer typically better performance compared to the type-II VMM. Examples of Type-I VMM include VMWare ESX (enterprise), Xen and Virtual Iron VFe.

Type-II

Type-II virtual machine monitors run on top of a host operating system which could be of any type (Linux, Windows, Unix,...). The type of operating systems that can be run as guest OS are independent of the choice of the host OS. Examples of Type-II VMM include VMWare GSX (workstation) and UML (User-Mode Linux)

VMM kernel

In the case of a Type-I VMM, there is a kernel embedded in the VMM that may or may not be a Linux kernel.

Host Kernel

This is the kernel that supports a Type-II VMM.

Guest kernel

This is the kernel running on a Virtual Machine.

Guest OS

This is the operating system stack running on a Virtual Machine.

Hard Partitioning

A type of system partitioning that provides electrical isolation between partitions of a single system, while allowing them to divide the resources on the system, such as i/o devices. The separation is physical and allows multiple Operating Systems to run by creating multiple physical layers.