Desktop Virtualisation

One type of desktop virtualisation is to use your desktop device only to display data while all the processing and storage (applications & data) is done by back-end servers.
Your desktop device could be a traditional PC or a specialised device like SunRay virtual display Client.

To implement this type of desktop virtualisation, you need to run a virtual desktop infrastructure Software on the desktops.

There are several virtual desktop infrastructure Software like:

• VMware's Virtual Desktop Infrastructure.
• Sun VDI Software.

Benefits of Implementing this type of Desktop Virtualisation:

• Data is stored in a centralised location for easy management and is less likely to be lost or stolen.
• Huge amounts of savings from reduced desktop system administration.
• Reduced IT maintainance, upgrades, and threats from viruses and other malware.

Another type of desktop virtualisation is to run a software which enables you install different operating systems on your desktop.

You could have a system running windows and you install this software which enables you run Linux on the same computer.

Data/Storage Virtualisation

Managing disk storage was once simple: If we needed more space, we got a bigger disk drive. But data storage needs grew, so we started adding multiple disk drives. Finding and managing these became harder and took more time, so we developed RAID, network-attached storage and storage-area networks. Still, managing and maintaining thousands of disk drives became an ever more onerous task.

The latest answer to this dilemma is storage virtualisation, which adds a new layer of software and/or hardware between storage systems and servers, so that applications no longer need to know on which specific drives, partitions or storage subsystems their data resides. Administrators can identify, provision and manage distributed storage as if it were a single, consolidated resource. Availability also increases with storage virtualisation, since applications aren't restricted to specific storage resources and are thus insulated from most interruptions.

Also, storage virtualisation generally helps automate the expansion of storage capacity, reducing the need for manual provisioning. Storage resources can be updated on the fly without affecting application performance, thus reducing downtime.

Server Virtualisation

There are three types of server virtualisation - operating system virtualisation, hardware emulation & paravirtualisation.

Operating System (OS) Virtualisation - also known as "Containers" enables an operating system ("guest") to run on top a "host" operating system. The guest operating system makes the resources of the hardware on which it is installed available to the applications using it. The applications have no interaction with the host operating system. In fact according to these applications, they are the only one interacting with the hardware.

You can use this type of virtualisation (container virtualisation) to offer different operating systems to different users, with a single physical machine.

This is the ideal type of virtualisation for web hosting companies. They can host several different websites on the same physical machine, with each website having its own "container". To each website, they are in total control of the machine, but in reality they are sharing it with other websites.

One serious drawback of operating system virtualisation is that your choice operating system is limited depending on the host operating system.
Example operating system virtualisation:
Sun - Solaris Operating System.

Hardware Emulation
With this type of virtualisation, the virtualisation software also known as hypervisor serves up an emulated hardware environment for guest operating system to operate on. The emulated hardware environment is called a virtual machine monitor. I.e. the virtualisation software (hypervisor) "fools" the guest operating system, into thinking it has a real hardware environment on which to operate, by presenting it the virtual environment known as virtual machine monitor (VMM).

The hypervisor sits between the VMM and the physical hardware and acts as an interpreter between the two. Each guest OS runs on one VMM
This implementation implies multiple OS and also different types of Operating software can run on the same machine. For example you can run windows and Linux on the same machine or different versions of windows on the same physical machine.

Software development companies can use this type of virtualisation to test their software on different operating systems, without having to buy new machines for each operating system.
You can also use hardware emulation virtualisation to move your applications environments unto the same physical machine.

The major disadvantage of hardware emulation is that the hypervisor (virtualisation software) hurts performance, and you will often find that applications run slower on virtualized systems.
One other drawback is that since the VMM acts as an interpreter between the hypervisor and the physical machine, there is need for device drivers to be installed between the hypervisor and the VMM, however they is need to update these drivers from time to time. But the users are unable to install these devices. This might lead to a situation where some resources may not run on virtualized environment when there are hypervisor drivers for them.

Where to get hardware emulation virtualisation (hypervisor software):
VMware - VMware Server & ESX Server
Microsoft - VMware (supports X86 servers only, emphasis on MS OS), Hyper-V,
Xen - Open source alternative

Paravirtualisation
With a paravirtualisation implementation, the virtualisation software sits between the guest operating system and the resources of the physical machine. The virtualisation software controls access to the resources of the physical machine.

The main advantage of this implementation regime is that there is less performance overhead used. There is also no need for device drivers like is the case with hardware emulation.
It has its own drawbacks which include the fact that

Examples:
Xen from XenSource (found in Red Hat and Novell distributions
Virtual iron also from Xensource

How virtualisation works...

To understand how virtualisation works, it is better perhaps to look at the different types of virtualisation. Each works in a different way and is implemented differently.

There are three different types of virtualisation: Server Virtualisation, Data or Storage Virtualisation and Desktop Virtualisation.