If hypervisor is commodity, why is VMware still on top?

KVM

KVM falls somewhere in between the two commercial behemoths. While Xen is the darling of Citrix, Amazon, Rackspace and a few others, KVM has clearly become the default open source hypervisor. This is in large part because Red Hat declared that it should be so, and thus it was.

A great example of this is any discussion involving OpenStack. When normal people talk about OpenStack they are talking about a series of open-source storage and networking projects attached to the KVM hypervisor and some management tools.

You can use OpenStack with other hypervisors, but doing so will draw some stares and mutterings of "oh, he's one of those people". Which is odd, because OpenStack is well into "people who keep pee in jars" territory all on its own, so getting finicky about the level of bizarreness when you're that far down the rabbit hole is perhaps a little counter-productive.

KVM is a perfectly decent hypervisor. It's fast, it does all the basic things you want a hypervisor to do and most of the tricky advanced stuff, too. The problem – and this is true of most of the open-source world – lies in actually implementing and managing the thing.

The default KVM management tools that ship with most Linux distributions are designed by people who think "esoteric uses of bash syntax" is great fun at parties. Microsoft may have infected the world with the ribbon and whatever the hell Metro was supposed to be, but virt-manager should be considered a war crime.

A list of KVM management tools can be found here. The existence of this list is both the explanation for why KVM hasn't taken over the world and the reason it is wrecking Xen.

The fact that there are so many different interfaces for KVM – and that list doesn't cover even half of what's out there – is KVM's Achilles' heel. It's virtually impossible to have a "shared experience" with KVM. There is no loyalty to the hypervisor itself or any of the affiliated projects. Administrators become members of the community around the management tools or virtualization appliances, and those tools eventually evolve to handle multiple hypervisors, containers, public clouds and so forth.

The best-balanced management tool on the KVM list is probably Proxmox, and like 5Nine, it's basically become a cult. The coolest is probably Mist.io, which can manage pretty much anything that needs managing all from a neat cloudy interface. Awesome possum, except for that whole "giving an American company access to the servers/cloud accounts that my entire business runs on" thing.

Xen

Xen can be summed up by visiting this page. The page is a nice, simple, clean wiki that lists various Xen management tools. At the top is a banner telling visitors "for the latest information, see the Ecosystem Directory on XenProject.org." The XenProject.org site is... well, it's not anywhere near as easy to use as that wiki.

Xen is the sort of thing that's fine and good if you have a warehouse full of PhDs and an IT budget rivaling the GDP of a small nation state. In a lot of ways it is technically superior to KVM. It's also fractious, with massively vested interests pulling the thing in a number of different directions and it suffers from a tragic lack of anything resembling centralised leadership.

For a time, Citrix was that leader, but Citrix had an uphill battle on its hands. The big US public clouds are Xen. From Amazon to Rackspace to IBM's Softlayer, there are powerful companies with vested interests in Xen. Citrix, with its focus on providing a usable Xen that customers installed in their own data centres – and with Citrix's unique focus on Terminal Services and VDI – was the odd man out.

Citrix is still a guiding force for Xen, but it has slowly had to cede control to the cloud behemoths. The results have been mixed. While the KVM ecosystem is absolutely exploding, Xen has really collapsed into occupying the sorts of niches that other hypervisors haven't taken over yet.

Remote application delivery and remote desktop delivery are great examples. Citrix ultimately open-sourced XenServer, but hung on to the bits that made money: everything around Citrix's core competency of VDI.