Hardware.

The term "virtualization" was coined in the 1960s, to refer to a virtual machine (sometimes called pseudo machine), a term which itself dates from the experimental IBM M44/44X system.[citation needed] The creation and management of virtual machines has been called platform virtualization, or server virtualization, more recently.

Platform virtualization is performed on a given hardware platform by host software (a control program), which creates a simulated computer environment, a virtual machine, for its guest software. The guest software is not limited to user applications; many hosts allow the execution of complete operating systems. The guest software executes as if it were running directly on the physical hardware, with several notable caveats. Access to physical system resources (such as the network access, display, keyboard, and disk storage) is generally managed at a more restrictive level than the processor and system-memory. Guests are often restricted from accessing specific peripheral devices, or may be limited to a subset of the device's native capabilities, depending on the hardware access policy implemented by the virtualization host.

Reasons for virtualization.

n 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.

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.[3]

A new virtual machine can be provisioned as needed without the need for an up-front hardware purchase. Also, 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.

There are several approaches to platform virtualization.

Full virtualization

In full virtualization, the virtual machine simulates enough hardware to allow an unmodified "guest" OS (one designed for the same instruction set) to be run in isolation. This approach was pioneered in 1966 with the IBM CP-40 and CP-67, predecessors of the VM family. Examples outside the mainframe field include Parallels Workstation, Parallels Desktop for Mac, VirtualBox, Virtual Iron, Oracle VM, Virtual PC, Virtual Server, Hyper-V, VMware Workstation, VMware Server (formerly GSX Server), QEMU, Adeos, Mac-on-Linux, Win4BSD, Win4Lin Pro, and Egenera vBlade technology.

Hardware-assisted virtualization

In hardware-assisted virtualization, the hardware provides architectural support that facilitates building a virtual machine monitor and allows guest OSes to be run in isolation [4]. Hardware-assisted virtualization was first introduced on the IBM System/370 in 1972, for use with VM/370, the first virtual machine operating system. In 2005 and 2006, Intel and AMD provided additional hardware to support virtualization. Examples of virtualization platforms adapted to such hardware include Linux KVM, VMware Workstation, VMware Fusion, Microsoft Virtual PC, Xen, Parallels Desktop for Mac, VirtualBox and Parallels Workstation.