In general there are different types of virtualization like Memory,CPU, Storage, Hardware and Network virtualization. Howver here we are going to talk about OS virtualization only.
1] What is Hypervisor -
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.
The words host and guest are used to distinguish the software that runs on the physical 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 Manager.
2] Type of Virtualization -
a) Para-virtualization -
-Guest OS has to be modified
-VM does not simulate hardware
-Use special API that a modified guest OS must use
-Hypercalls trapped by the Hypervisor and serviced
-Xen, VMWare ESX Server
b) Full-virtualization (Native) -
VM simulates “enough” hardware to allow an unmodified guest OS to be run in
isolation. Same Hardware and CPU/Memory, eg- Vmware,IBM VM family,Parallels,
* Full virtualization with Xen Hypervisor requires:
i) Intel processor with the Intel VT extensions, or
ii) AMD processor with the AMD-V extensions, or
iii) an Intel Itanium processor
* Full virtualization with KVM hypervisor requires:
i) Intel processor with the Intel VT and the Intel 64 extensions, or
ii) AMD processor with the AMD-V and the AMD64 extensions
c) Emulation -
-VM emulates/simulates complete hardware
-Unmodified guest OS for a different PC can be run
-VirtualPC for Mac, QEMU
d) OS-level virtualization -
-OS allows multiple secure virtual servers to be run
-Guest OS is the same as the host OS, but appears isolated apps see an
isolated OS. eg: Solaris Containers, BSD Jails, Linux Vserver,OpenVZ and LXC (LinuX Containers)
e) Application level virtualization -
-Application is gives its own copy of components that are not shared
(eg: own registry files, global objects) – VE prevents conflicts, JVM