Xen was a standalone hypervisor that run under the Linux kernel. It was developed independently of Linux. Linux was a guest along for the ride.<p>With KVM the hypervisor is over the Linux kernel and gets to use the facilities of the host kernel. Less code, less complexity. Why wouldn't Linux kernel hackers prefer KVM over Xen? They get full control (aka the ability to get crap fixed without hounding an external party) and get to reuse code they already have to write anyway.<p>Preferring the KVM approach over Xen makes 100% sense to me in the long term for the Linux folks.
Xen is a classic example of a technology and company that never reached its full market potential due to nerd pride.<p>In this particular case, Redhat/Linus/kernel hacker's obstinate refusal to add it to the stock kernel. They exhibited NIH and did KVM (which surprise surprise was owned by Redhat). I'm no virtualization expert, but KVM is doing things totally different than everybody else (no hypervisor), and I'm not 100% sure that KVM's way is better.<p>Xen should be a cautionary tale for startups that opening source is not a silver bullet. The capricious behavior of a coterie can kill you even if that coterie is ostensiby not controlled by any company.