How Hypervisors Work

A hypervisor is a software system that partitions a single physical machine into multiple virtual machines, to provide server consolidation and utility computing. Existing applications and binaries run unmodified.

The hypervisor presents a virtual machine to guests. The hypervisor forms a layer between the software running in the virtual machine and the hardware. This separation enables the hypervisor to control how guest operating systems running inside a virtual machine use hardware resources.

Uniform View of Hardware

A hypervisor provides a uniform view of underlying hardware. Machines from different vendors with different I/O subsystems appear to be the same machine, which means that virtual machines can run on any available supported computer. Thus, administrators can view hardware as a pool of resources that can run arbitrary services on demand. Because the hypervisor also encapsulates a virtual machine's software state, the hypervisor layer can map and remap virtual machines to available hardware resources at any time and also use live migration to move virtual machines across computers. These capabilities can also be used for load balancing among a collection of machines, dealing with hardware failures, and scaling systems. When a computer fails and must go offline or when a new machine comes online, the hypervisor layer can remap virtual machines accordingly. Virtual machines are also easy to replicate, which allows administrators to bring new services online as needed.

Using Domain Capabilites

Containment

Containment gives administrators a general-purpose undo capability. Administrators can suspend a virtual machine and resume it at any time, or checkpoint a virtual machine and roll it back to a previous execution state. With this capability, systems can more easily recover from crashes or configuration errors. See Recovery.

Containment also supports a very flexible mobility model. Users can copy a suspended virtual machine over a network or store and transport it on removable media. The hypervisor provides total mediation of all interactions between the virtual machine and underlying hardware, thus allowing strong isolation between virtual machines and supporting the multiplexing of many virtual machines on a single hardware platform. The hypervisor can consolidate several physical machines with low rates of utilization as virtual systems on a single computer, thereby lowering hardware costs and space requirements.

Security

Strong isolation is also valuable for reliability and security. Applications that previously ran together on one machine can now be separated on different virtual machines. If one application experiences a fault, the other applications are isolated from this occurrence and will not be affected. Further, if a virtual machine is compromised, the incident is contained to only that compromised virtual machine.