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I was interested in experimenting with Xen until I discovered that it has to be installed bare metal. Hence, if I have a Linux machine and want to use it to run a Xen hypervisor, I have to install the Xen server/kernel on the machine bare metal, and turn my host Linux into a VM of the Xen server!

I have used VirtualBox a lot before, where VBox sits "on top" of my host OS, allowing me to keep my Linux/Windows/whatever host, and run the hypervisor off of it.

I am wondering: which department does KVM fall into? Is KVM like Xen, where I have to install it bare metal, or can it be installed "on top of" an existing host (Windows/Linux) OS?

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Please do at least some basic research.

The Wikipedia article on KVM says that it is implemented as a Linux kernel module (and later ported to Illumos), thus mostly type-2 like VirtualBox. But since KVM comes as a standard part of the OS, you could also say it's right in the middle.

(However, you still need to install the user-space tools (qemu -enable-kvm) to use it on Linux.)

Or, as the hypervisor classification page says,

… the distinction between these two types is not necessarily clear. Linux's Kernel-based Virtual Machine (KVM) and FreeBSD's bhyve are kernel modules that effectively convert the host operating system to a type-1 hypervisor.

Nevertheless, since Linux distributions and FreeBSD are still general-purpose operating systems, with other applications competing for VM resources, KVM and bhyve can also be categorized as type-2 hypervisors.

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  • Thanks @grawity (+1) - I did not realize KVM was specific to Linux. A quick followup if you don't mind: if Macs are based on FreeBSD, then is KVM available to Macs as a hypervisor option?
    – smeeb
    May 13, 2015 at 16:09
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    @smeeb: No. FreeBSD only implemented bhyve (its KVM equivalent) in 2014. While the OS X kernel (but not Mac OS) does have bits and pieces of FreeBSD, they're at least a decade older than that. You could use Parallels or VirtualBox on OS X, though.
    – user1686
    May 13, 2015 at 16:12
  • Thanks @grawity (+1 again), do you mean that I could use VirtualBox (which I'm familiar with) to spin upa KVM hypervisor, is that what you're implying?
    – smeeb
    May 13, 2015 at 16:14
  • I'm saying that VirtualBox is a hypervisor; it does not use KVM though, and OS X doesn't have KVM.
    – user1686
    May 13, 2015 at 16:15

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