Virtualization is achieved through the software known as virtual machine monitor or the hypervisor . the software is used in two ways thus forming two different structure of virtualization,namely Bare Metal Virtualization and Hosted Virtualization.

Bare-metal virtualization hypervisors: (TYPE I HYPERVISOR)

• Is deployed as a bare-metal installation (the first thing to be installed on a server as the operating system will be the hypervisor).

• The hypervisor will communicate directly with the underlying physical server hardware, manages all hardware resources and support execution of VMs.

• Hardware support is typically more limited, because the hypervisor usually has limited device drivers built into it.

• Well suited for enterprise data centers, because it usually comes with advanced features for resource management, high availability and security.

• Bare-metal virtualization hypervisors examples: VMware ESX and ESXi, Microsoft Hyper-V, Citrix Systems XenServer.

Hosted virtualization hypervisors: (TYPE II HYPERVISOR)

• The software is not installed onto the bare-metal, but instead is loaded on top of an already live operating system, so it requires you to first install an OS(Host OS).

• The Host OS integrates a hypervisor that is responsible for providing the virtual machines(VMs) with their virtual platform interface and for managing all context switching scheduling, etc.

• The hypervisor will invoke drivers or other component of the Host OS as needed.

• On the Host OS you may run Guest VMs, but you can also run native applications

• This approach provides better hardware compatibility than bare-metal virtualization, because the OS is responsible for the hardware drivers instead of the hypervisor.

• A hosted virtualization hypervisor does not have direct access to hardware and must go through the OS, which increases resource overhead and can degrade virtual machine (VM) performance.

• The latency is minimal and with today’s modern software enhancements, the hypervisor can still perform optimally.

• Common for desktops, because they allow you to run multiple OSes. These virtualization hypervisor types are also popular for developers, to maintain application compatibility on modern OSes.

• Because there are typically many services and applications running on the host OS, the hypervisor often steals resources from the VMs running on it

• The most popular hosted virtualization hypervisors are: VMware Workstation, Server, Player and Fusion; Oracle VM VirtualBox; Microsoft Virtual PC; Parallels Desktop.

• The below figure shows stucture of TYPE I and TYPE II virtualization.