Type 1 and Type 2 Hypervisors Explained

In a recent discussion, the topic of Type 1 and Type 2 hypervisors came up. To some, this is an arbitrary distinction that doesn't matter much as there's already an inherent understanding of what the requirements are for a virtualization solution.

Simply put, the distinction between Type 1 and Type 2 has to do with whether an underlying operating system is present. Virtualization Review editor Keith Ward touched on part of this topic in a post about KVM virtualization in regards to Red Hat.

I'm convinced there's no formal standards-based definition of Type 1 and Type 2 criteria. However, I did like this very succinct piece of literature from IBM. While it doesn't have an exhaustive list of hypervisors and their types, it does give good definitions. The material describes a Type 1 hypervisor as running directly on the hardware with VM resources provided by the hypervisor. The IBM Systems Software Information Center material further states that a Type 2 hypervisor runs on a host operating system to provide virtualization services.

Some are obvious, such as VMware ESXi and Citrix XenServer being Type 1 hypervisors. My beloved Sun VirtualBox, VMware Server and Microsoft Virtual PC are all Type 2 hypervisors.

With that said, it's unclear where Hyper-V fits into the mix. Information like this Microsoft virtualization team blog post pull Hyper-V closer to the Windows Server 2008 base product.

The relevance of Type 1 and Type 2 distinction is academic, in my opinion, but something I wanted to share here. Your thoughts? Drop me a note or share a comment below.

Posted by Rick Vanover on 06/24/2009 at 12:47 PM