Virtualization allows numerous instances of guest operating systems to run on a single physical machine ( host machine ),
and this is the basis for innovative web hosting offerings, such as cloud computing and Virtual Private Servers.
There are many reasons for using virtualization.

Here are some the features of Virtualization :

System Consolidation : By using virtualisation you can install and combine multiple software on single system. You can run various virtual machines in order to save power, space, cooling needs.

Technology Testing : It provides way for developers to test and debug their software on multiple guests. You can test various application configurations on same machine as per priority and resources. This is best solution for students who wish to learn new operating systems and programming languages / database without making any changes to working environment.

System Administration : If you have single server running multple OS then it reduces operating expenses, one system administrator can manage two servers.

Security and Isolation
: If a guest OS running on a VM becomes infected with malware, the host operating system’s exposure to the risk may be limited, depending on the configuration of the virtualization software.

Virtualization is a vast term that refers to the abstraction of computer resources such as,Platform Virtualization,Resource Virtualization,Desktop Virtualization, Storage Virtualization, Network Virtualization.



Freely available ( Open Source ) Linux Virtualization Software :

User Mode Linux (UML):

It was the first virtualization technology for Linux. User-mode Linux is generally considered to have lower performance than some competing technologies, such as Xen and OpenVZ. Future work in adding support for x86 virtualization to UML may reduce this disadvantage.

The User-mode Linux Kernel Home Page
User-mode Linux – Wikipedia, the free encyclopedia

Linux-VServer :

Linux-VServer is a virtual private server implementation done by adding operating system-level virtualization capabilities to the Linux kernel.

Linux-VServer
Linux-VServer – Wikipedia, the free encyclopedia

OpenVZ :

OpenVZ is container-based, operating system-level virtualization for Linux. OpenVZ creates multiple secure, isolated containers on a single physical server enabling better server utilization and ensuring that applications do not conflict.

OpenVZ Wiki
OpenVZ – Wikipedia, the free encyclopedia

Xen :

Xen is a virtual-machine monitor for IA-32, x86-64, Itanium and PowerPC 970 architectures, It allows several guest operating systems to be executed on the same computer hardware concurrently. XEN is included in most popular Linux distributions such as Debian, Ubuntu, CentOS, RHEL, Fedora and many others.

Welcome to xen.org, home of the Xen® hypervisor, the powerful open source industry standard for virtualization.
Xen – Wikipedia, the free encyclopedia

KVM :

KVM ( Kernel-based Virtual Machine ) is a Linux kernel virtualization infrastructure. This currently supports native virtualization using Intel VT or AMD-V. A wide variety of guest operating systems work with KVM, including many flavours of Linux, BSD, Solaris, and Windows etc.

Main Page – KVM
Kernel-based Virtual Machine – Wikipedia, the free encyclopedia

VirtualBox :

VirtualBox is an x86 virtualization software package, developed by Sun Microsystems as part of its Sun xVM virtualization platform. Supported host operating systems include Linux, Mac OS X, OS/2 Warp, Windows XP or Vista, and Solaris, while supported guest operating systems include FreeBSD, Linux, OpenBSD, OS/2 Warp, Windows and Solaris.

VirtualBox
VirtualBox – Wikipedia, the free encyclopedia


Enterprise ( licenced ) Linux Virtualization Software :

Parallels Virtuozzo Containers:

It is an operating system-level virtualization product designed for large-scale homegenous server environments and data centers. Parallels Virtuozzo Containers is compatible with x86, x86-64 and IA-64 platforms. You can run various Linux distributions inside Parallels Virtuozzo Containers.

Virtualization & Automation Solutions for Desktops, Servers, Hosting, SaaS ? Parallels Optimized Computing
http://en.wikipedia.org/wiki/Parallels,_Inc.

VMWare ESX Server and VMWare Server:

VMware ESX Server is an enterprise-level virtualization product providing data center virtualization. It can run various guest operating systems such as FreeBSD, Linux, Solaris, Windows and others.

VMware Virtualization Software for Desktops, Servers & Virtual Machines for a Private Cloud
VMware Server – Wikipedia, the free encyclopedia

Enterprise level XEN implementations with additional features and support.

Citrix XenServer :

XenServer is the enterprise-ready, cloud-proven virtualization platform that contains all the capabilities required to create and manage a virtual infrastructure.
It is based on the open source Xen hypervisor, an exceptionally lean technology that delivers low overhead and near-native performance.
Citrix Systems – Wikipedia, the free encyclopedia
Citrix Systems – Virtualization, Networking and Cloud. Simplified.

Oracle VM :

Oracle VM is a free, next-generation server virtualization and management solution from Oracle that makes enterprise applications easier to deploy, manage, and support. Users can create and manage Virtual Machines (VMs) that exist on the same physical server but behave like independent physical servers. Each virtual machine created with Oracle VM has its own virtual CPUs, network interfaces, storage and operating system.
Oracle VM is based on the open-source Xen hypervisor technology, supports both Windows and Linux guests and includes an integrated Web browser based management console.

Oracle VM – Wikipedia, the free encyclopedia
Oracle and Virtualization | Oracle Virtualization

Sun xVM :

Sun xVM is a product group from Sun Microsystems that addresses virtualization technology on x86 platforms. The xVM Server uses a bare-metal hypervisor based on the open source Xen under a Solaris environment. On SPARC systems, xVM is based on Sun’s Logical Domains and Solaris. Sun plans to support Microsoft Windows (on x86-64 systems only), Linux, and Solaris as guest operating systems.

VM VirtualBox | Virtualization | Oracle
Sun xVM – Wikipedia, the free encyclopedia

References and sources : wikipedia and company portals.