Clouds are based on virtualization since it provides simplicity of building and using distributed environment without creating dependencies on physical resources

Types of Virtualization:

1. Native, full
2. Hardware assisted
3. Para-virtualization (such as XEN)
4. OS level, including: Containers, Jails, Chroot, Zones, etc.

Native virtualization: the virtual machine stimulates enough hardware to allow an UNMODIFIED guest OS

Hardware assisted : the virtual machine has its own hardware (such CPU or GPU) and allows a guest OS to be run in isolation.

Para-virtualization : the virtual machine does not stimulate hardware and instead offers a special API that can only be used by modifying the “guest” OS

OS Level virtualization: virtualizing a physical server, and allow multiple virtual servers to fun on a single physical server above the OS level.