It is creation of a virtual rather than actual version of something,such as a hardware
platform, operating system, a storage device or network resources.
For example an operating system virtualization is the use of software to allow a piece of
hardware to run multiple operating system images at the same time. The technology got
its start on mainframes decades ago, allowing administrators to avoid wasting expensive
processing power.

Types of virtualization

1. Hardware virtualization
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
In hardware virtualization, the term host machine refers to the actual machine on which
the virtualization takes place; the term guest machine, however, refers to the virtual
machine. Likewise, the adjectives host and guest are used to help 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.
Improving the efficiency of hardware virtualization is known as Hardware-assisted

2. Software virtualization
It includes Operating system-level virtualization which is hosting of multiple
virtualized environments within a single OS instance and Application virtualization
and Workspace virtualization, the hosting of individual applications in an environment
separated from the underlying OS

3. 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

4. Data virtualization
Data virtualization is 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

5. Storage virtualization
Storage virtualization is the process of completely abstracting logical storage from
physical storage.
Distributed file system : Any file system that allows access to files from multiple hosts
sharing via a computer network.[1] This makes it possible for multiple users on multiple
machines to share files and storage resources.

6. Network virtualization
Creation of a virtualized network addressing space within or across network subnets.
Desktop virtualization is a concept of separating a desktop environment from its physical
computer (and its associated operating system) and storing it on another machine across a
network, such as a center server. Thin clients employ desktop virtualization.


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