Virtualization Essentials---Understanding hypervisor

Original link from : http://searchservervirtualization.techtarget.com/tip/Understanding-hosted-and-bare-metal-virtualization-hypervisor-types

A virtualization hypervisor comes in one of two forms: a bare-metal hypervisor, also known as Type 1; or a hosted hypervisor, also known as Type 2. There are important differences between a hosted and bare-metal virtualization hypervisor, and each has pretty specific use cases.

A bare-metal virtualization hypervisor does not require admins to install a server operating system first. Bare-metal virtualization means the hypervisor has direct access to hardware resources, which results in better performance, scalability and stability. One disadvantage of a bare-metal virtualization hypervisor, however, is that hardware support is typically more limited, because the hypervisor usually has limited device drivers built into it. (所以说为什么azure的私有云只支持联想和华为的一些硬件设备。)

are-metal virtualization  is well suited for enterprise data centers, because it usually comes with advanced features for resource management, high availability and security. Admins can centrally manage this kind of virtualization hypervisor, which is critical when you have many hosts in your virtual infrastructure. The most popular bare-metal virtualization hypervisors are:

• VMware ESX and ESXi
• Microsoft Hyper-V
• Citrix Systems XenServer

Unlike the bare-metal virtualization hypervisor, a hosted hypervisor requires you to first install an OS. These hypervisors are basically like applications that install on a guest OS. This approach provides better hardware compatibility than bare-metal virtualization, because the OS is responsible for the hardware drivers instead of the hypervisor.

But, as with the bare-metal hypervisor, there are disadvantages. 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. Also, because there are typically many services and applications running on the host OS, the hypervisor often steals resources from the VMs running on it.

Hosted hypervisors are 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. The most popular hosted virtualization hypervisors are:

• VMware Workstation, Server, Player and Fusion
• Oracle VM VirtualBox
• Microsoft Virtual PC
• Parallels Desktop

Once you understand the differences between a hosted and bare-metal virtualization hypervisor -- and the best use cases for each -- it’s time to consider hypervisor vendors, cost and features.