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If hypervisor is commodity, why is VMware still on top?
Microsoft and open-source rivals lose it in the labs
Xen and the art of hyperscale management
Similarly, Xen hangs on stubbornly at hyperscale, but mostly because "it was there first". KVM has proven itself at scale – HP's Helion cloud is an example. But the Amazons of the world are not going to go tear up their investment in Xen for KVM, especially when they have such a strong say in the future of Xen, but KVM's future is largely dictated by Red Hat.
The result is two open-source hypervisors with two very different futures. KVM is evolving to become the open-source replacement for ESXi and Hyper-V. The ecosystem around it is all about commercial implementations by companies ranging from SMBs to Enterprises, and increasingly organisations looking to stand up regional clouds.
Xen is, well, Xen. It's an afterthought to a lot of companies. It is becoming its own thing used for its own purposes. Purposes that most companies aren't going to care about, which explains the lack of ecosystem interest.
This isn't to say that there aren't groups out there pushing to have Xen considered for the same applications as ESXi, Hyper-V and KVM. It's just that they're rather bad at politicking and marketing, with the predictable results of patchy and inconsistent success.
Most companies will have a good long think about ESXi versus Microsoft. Especially if they are starting a new virtualisation project or are spinning up a new company. Increasingly, KVM will be a part of that conversation, but it's still really rare to hear of Xen seriously discussed here.
On the other hand, companies that are otherwise entirely VMware or Hyper-V will absolutely bring in Citrix-based Xen servers to handle VDI deployments, treating them essentially as a separate environment from the rest of the virtualisation infrastructure.
ESXi, Hyper-V and KVM are infrastructure solutions. You build yourself some infrastructure made out of one of these three then load it up with VMs. But the focus is on the infrastructure itself, in no small part because of all the companies in the associated ecosystems wanting to sell you solutions to various infrastructure problems.
Xen is a point solution. If you have a specific problem, there may be a Xen-based solution, but there isn't a whole lot of "build out Xen infrastructure and then waiting to see what gets put on it". Except, you know, for virtually the entire public cloud.
ESXi, Hyper-V and KVM are at this point functionally interchangeable. Every now and again one of them experiences a generational leap in capability. Currently, VMware has the lead with the vSphere 6.0 family of products, but that won't last long.
Ultimately, it keeps coming back to those management tools. Xen has trouble bridging the gap and taking over general infrastructure because of a lack of quality management tools and confusion about licensing the ones that exist.
KVM lags Hyper-V mostly due to a historicity of bad management tools and a current state of too many management tools. Hyper-V lags ESXi because System Center was sent from hell to make us miserable.
All of which really boils down to one key take home: when it comes to hypervisors, the tech is solid all over. It's ease of use that matters – and management tools, of course. ®