KVM is one of those things I've kept an eye open for ever since I heard about it. First of all, I should mention that there is currently a great deal happening on the virtualization front, here's a list of virtual machines to look over. What is more, it is not so much a wealth of competing products, because it seems that everyone has their own little twist on the matter, and none of these are completely equivalent solutions. There is a host of different terms to describe these techniques to boot.

I've known about VMware for 5-6 years, and that is how I originally started out playing around with linux in a safe and easy environment. Since then there have been many linux centric projects that have made waves at some time or another, not all of which I've tried. I think User Mode Linux was the first one I heard about. It ran a modified host kernel and could host multiple guest machines that would also be running a modified kernel. You could use it to host Virtual Private Servers, which has been common now for a while, but wasn't all that common back then. You could also set up honeypots with them. There was also a VMware competitor called Win4Lin (for Win9x) which was like a poor man's VMware. It would run fairly well, but it was far from being equally polished and feature complete. It also had the advantage of being strictly an application, no kernel hooks. A couple of years later there was a new project that was very loud, namely Xen. Xen is one of those odd projects that had a lot of potential and broad support, and yet didn't quite have the kind of adoption one would expect. It was quite a complicated piece of code that required a lot of setting up and so on, but Fedora shipped it by default (I'm not sure if they still do) and it worked out of the box. Again it was a modified host kernel, but the guests could run completely unmodified. I'm not sure how recent this is, but nowadays Xen also runs various operating systems, although in some cases they need to be modified. Then there was CoLinux, which approached the issue from the opposite side of the table: Linux as guest. I never got around to trying it as I never had to use Windows for long stretches of time, but a lot of people (those poor souls trapped on Windows) were very enthusiastic about it.

So what about KVM? Well, KVM is interesting because of how "close to the metal" it is. Obviously, any kind of virtualization adds some overhead due to the unavoidable indirection. But KVM actually does not position itself above the kernel, as most virtual machines do, but uses kernel primitives to host guests (in fact it sets up a whole new kernel execution mode guest for this). It also uses cpu virtualization extensions (current Intel and AMD chips) to gain speed. All of this is fairly recent stuff, KVM was merged into the kernel for 2.6.20. So, of course, the major advantage of KVM is that it's well supported and well tested in kernel mainline.

However, KVM is not a virtual machine in itself, in the sense of being a complete application. It's more like an access layer which exposes a /dev/kvm interface. The way to run KVM is through QEMU, which is actually an emulator. I haven't mentioned emulators so far, but they are in a sense the second type of virtualizer/emulator software. The difference is that a virtualizer creates a virtual machines with virtual hardware (eg. network adapter) for the guest operating system to run on, but the cpu is still that of the host. An emulator instead emulated the guest cpu, and therefore has to translate every machine instruction into a different one. This means that you can run say a powerpc guest on an x86 host. It sounds very cool, but it's orders of magnitude slower, so it's not as much of a hot topic.

But back to KVM. As I said, the application is still QEMU, but it's accelerated through KVM. I had been looking forward to taking it for a spin, especially now that I also have an Intel chip with the virtualizing extensions. On Ubuntu it's all ready to go, just install the userland applications from the repo (qemu and kvm) and you're set. Qemu is quite nice and simple to use, but the combo is temperamental and unforgiving. You can set up your guest machine in 2 minutes, but with Windows I had quite a number of fatal crashes (Exception 13) that aren't that obvious to figure out. Furthermore, it seems that even linux distros aren't trivial to run on KVM/QEMU. In terms of a complete machine virtualization it leaves something to be desired. Notably, the video flickers quite visibly while running on a vesa driver. That isn't to say that other virtual machines cannot or will not use KVM as part of their solution.

But for the time being VirtualBox (something of a lightweight VMware, which is also free software) is more convenient.

Disclaimer: This entry is something of a historical account of my exposure to virtual machines. I did not do any fact checking here, and the statements only reflect what I recall about the particular products.