"In the mean time, KVM showed up and grabbed much of the attention. Its path into the mainline was almost blindingly fast, and many kernel developers were less than shy about expressing their preference for the KVM approach. More recently, Red Hat has made things more formal with its announcement of a "virtualization agenda" based on KVM. Meanwhile, lguest showed up as an easy introduction for those who want to play with virtualization code.
"The Xen story is a classic example of the reasons behind the "upstream first" policy, which states that code should be merged into the mainline before being shipped to customers. Distributors rushed to ship Xen, then found themselves supporting out-of-tree code which, often, was not well supported by its creators. In particular, published releases of Xen often only supported relatively old kernels, creating lots of work for distributors wanting to ship something more current. Now at least some of those distributors are moving on to other solutions, and high-level kernel developers are questioning whether, at this point, it's worth merging the remaining Xen code at all."
