A hypervisor is a program that would enable you to host several different virtual machines on a single physical machine. These virtual machines “think” and “act” like physical machines and can be altered to more efficiently distribute the available resources.
Several virtual machines can live on a single physical machines, sharing resources as needed. The main role of hypervisor is to allocate available hardware resources to the virtual machines.
We have two major classes of hypervisors and they are defined as: type 1 and type 2.
Type 1 hypervisors are commonly considered as bare metal hypervisors, in that the hypervisor software itself runs directly on top of your hardware. Type 1 hypervisors tend to enjoy much better performance than type 2 hypervisors, due in part to their direct positioning on top of hardware.
A type 2 hypervisors are commonly considered as hosted hypervisors and must be installed on top of an existing operating system. A type 2 hypervisor is great for testing and evaluation situations not so much for production purposes.
List of free Virtualization Software & Hypervisors:
Xen Hypervisor (Type 1 hypervisor) is an Open Source virtualization software designed to virtualize x86 and x64 server architectures. Xen supports Hardware Assisted Virtualization and Paravirtualization.
Oracle VirtualBox (Type 2 hypervisor) is a cross-platform virtualization application. Oracle VirtualBox supports Hardware Assisted Virtualization and Paravirtualization.
Proxmox VE (Type 1 hypervisor) is a powerful Open Source server virtualization platform, based on KVM and OpenVZ.
Linux-VServer (Type 2 hypervisor) provides virtualization for GNU/Linux systems. This is accomplished by kernel level isolation. It allows to run multiple virtual units at once.
KVM (Kernel-based Virtual Machine) (Type 2 hypervisor) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V).
OpenVZ Linux Containers (Type 2 hypervisor) is container-based virtualization for Linux.