Types of Virtualization / Types of Hypervisors

This blog post will explain the types of server virtualization that is prevalent today and the various types of hypervisors ( the virtualization layer) in scope today.

Types of Virtualization:
1) FULL VIRTUALIZATION
This is also known as native virtualization. This method consists of a virtualization layer or a hypervisor that intermediates between the guest operating system and the underlying hardware.
For Example: VMWARE ESX SERVER .
This runs on top of a native hardware and intermedites the guest OS and the hardware. The hypervisor monitors the instructions executed by the guest OS and traps certain privileged instructions that directly meddle with the underlying hardware since it is shared across the guest OSes. The guest OS can run unmodified in the case of Full / Native Virtualization.

2) PARA VIRTUALIZATION
This type of virtualization still consists of a virtualization layer called the hypervisor to co-ordinate access to the shared hardware by multiple guest Operating Systems, but the guest OSes are themselved tuned to be virtualization aware so that there is no need of any trapping and hence close to performance of a OS running on top of a real hardware.
Ex: XenServer
The guest OS need to be modified for the hypervisor on which it will run.

Type of Hypervisors:
"Hypervisor" - The "visor" who manages multiple "Super" visors. In early days , each Operating system itself was termed as the "Super"visor since it manages the hardware and software resources. Now, since the hypervisor abstracts or manages the resources across multiple guest Oses or Supervisors , it is called as "Hypervisor".

1) Type 1 Hypervisor: Hypervisors that run directly on top of physical hardware are called as Type 1 or bare-metal hypervisors.
Ex: ESX Server , Citrix XenServer, Oracle VM, Microsoft Hyper-V
The guest OSes run at the second-level on top of the Type 1 hypervisors.
Hardware -> Type 1 Hypervisor -> Guest OS

2) Type 2 Hypervisor: Hypervisors that run within an Operating System Environment are called as Type 2 Hypersiors or Hosted Hypervisors.
Ex: Vmware Workstation, Microsoft Virtual PC
The guest Oses run at the third-level on top of the Type 2 hypervisors.
Hardware -> Base-OS -> Type-2 Hypervisor -> Guest OS.