Once upon a time, the only reliable virtualization products for the enterprise were expensive proprietary offerings like VMware. In recent years, that's changed as the virtualization market has diversified widely, with open-source virtualization solutions now readily available for enterprises seeking more flexibility or cost efficiency than closed-source vendors provide.
Xen vs. KVMXen, one of the first open-source virtualization hypervisors to become mature enough for production use, was endorsed by commercial Linux vendors like Red Hat and Suse from an early date. In contrast, Canonical, Ubuntu's parent company, has thrown most of its weight behind KVM, creating tools like ubuntu-vm-builder to simplify management of KVM environments.
These days, Xen and KVM are comparable in performance and features. KVM still has a bit of maturing to do on some fronts, but since it's integrated into the Linux kernel, it offers less overhead than Xen, as well as a simplified infrastructure.
Red Hat vs. UbuntuRed Hat's decision to offer official support for KVM likely stems from efforts to safeguard its enterprise market share against encroachment by Canonical, which continues to pour resources into developing a foothold on the Linux-server front. But whether the move will pay off for Red Hat remains to be seen.
On the one hand, support for KVM in RHEL 5.4 may convince some customers who would have switched to Ubuntu in order to take advantage of its virtualization tools to stick instead with Red Hat. But in order to keep those patrons, Red Hat needs to deliver a virtualization suite for KVM that's on par with the infrastructure already built into current Ubuntu releases.
That shouldn't be a difficult task for a huge vendor like Red Hat to accomplish, especially since the company already has sophisticated management tools like oVirt in place. Ubuntu's head start in the KVM realm, however, provides an important advantage that might be difficult to overcome--especially since Ubuntu server edition is available totally free of charge, while RHEL requires a license.
The one certainty is that KVM, and by extension all Linux distributions that support it, will benefit from Red Hat's integration, while the showdown between Ubuntu server edition and RHEL intensifies.