Virtualization, in computing, is the creation of a virtual (rather than actual) version of something, such as a hardware platform, operating system, a storage device or network resources.

Virtualization can be viewed as part of an overall trend in enterprise IT that includes autonomic computing, a scenario in which the IT environment will be able to manage itself based on perceived activity, and utility computing, in which computer processing power is seen as a utility that clients can pay for only as needed. The usual goal of virtualization is to centralize administrative tasks while improving scalability and work loads.

Hardware Virtualization:

Hardware virtualization or platform virtualization refers to the creation of a virtual machine that acts like a real computer with an operating system. Software executed on these virtual machines is separated from the underlying hardware resources.

In hardware virtualization, the host machine is the actual machine on which the virtualization takes place, and the guest machine is the virtual machine. The words host and guest are used to distinguish the software that runs on the actual machine from the software that runs on the virtual machine. The software or firmware that creates a virtual machine on the host hardware is called a hypervisor or Virtual Machine Monitor.

Different types of hardware virtualization include:

Full virtualization: Almost complete simulation of the actual hardware to allow software, which typically consists of a guest operating system, to run unmodified

Partial virtualization: Some but not all of the target environment is simulated. Some guest programs, therefore, may need modifications to run in this virtual environment.

Paravirtualization: A hardware environment is not simulated; however, the guest programs are executed in their own isolated domains, as if they are running on a separate system. Guest programs need to be specifically modified to run in this environment.

Hardware-assisted virtualization is a way of improving the efficiency of hardware virtualization. It involves employing specially-designed CPUs and hardware components that help improve the performance of a guest environment.

Hardware virtualization is not the same as hardware emulation: in hardware emulation, a piece of hardware imitates another, while in hardware virtualization, a hypervisor (a piece of software) imitates a particular piece of computer hardware or the whole computer altogether. Furthermore, a hypervisor is not the same as an emulator; both are computer programs that imitate hardware, but their domain of use in language differs.

Desktop Virtualization

Desktop virtualization is the concept of separating the logical desktop from the physical machine.

One form of desktop virtualization, virtual desktop infrastructure (VDI), can be thought as a more advanced form of hardware virtualization: Instead of directly interacting with a host computer via a keyboard, mouse and monitor connected to it, the user interacts with the host computer over a network connection (such as a LAN, Wireless LAN or even the Internet) using another desktop computer or a mobile device. In addition, the host computer in this scenario becomes a server computer capable of hosting multiple virtual machines at the same time for multiple users.

Another form, session virtualization, allows multiple users to connect and log into a shared but powerful computer over the network and use it simultaneously. Each is given a desktop and a personal folder in which they store their files.

Thin clients, which are seen in desktop virtualization, are simple and/or cheap computers that are primarily designed to connect to the network; they may lack significant hard disk storage space, RAM or even processing power.

Software Virtualization

Operating system-level virtualization, hosting of multiple virtualized environments within a single OS instance

Application virtualization and workspace virtualization, the hosting of individual applications in an environment separated from the underlying OS. Application virtualization is closely associated with the concept of portable applications.

Memory Virtualization

Memory virtualization, aggregating RAM resources from networked systems into a single memory pool. 

Virtual memory, giving an application program the impression that it has contiguous working memory, isolating it from the underlying physical memory implementation. 

Storage Virtualization

• Storage virtualization, the process of completely abstracting logical storage from physical storage
• Distributed file system
• Storage hypervisor

Data Virtualization

• Data virtualization, the presentation of data as an abstract layer, independent of underlying database systems, structures and storage

• Database virtualization, the decoupling of the database layer, which lies between the storage and application layers within the application stack

Network Virtualization

Network virtualization, creation of a virtualized network addressing space within or across network subnets

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Benefits of Virtualization (PDF)

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