Xen vs. KVM: Which open source hypervisor is ascendant?

Citrix has scored some success with Xen and Red Hat is championing KVM virtualization. Which is winning? Analysts say KVM needs some the wins to prove its viability in the market.

At the 2008 Red Hat Summit, Red Hat sent shock waves through its customer base and the open source community. At the time, it announced that it was switching its future development from the Xen virtualization engine built into Red Hat Enterprise Linux (RHEL) 5 to the newer, slimmer Kernel-based virtual machine(KVM) hypervisor embedded in the Linux kernel. Soon after, Red Hat cemented its virtualization switch by shelling out $107 million for Qumranet, KVM's creators, thereby capturing full developmental control.

Although Red Hat promised its customers to retain Xen in the OS through at least the end of RHEL 5 and to provide support for several years thereafter, Red Hat said its future development would focus on KVM. Today, KVM is included as a hypervisor along with Xen in the latest RHEL 5.4 release. Customers are free to choose one or the other as part of the installation process and, essentially, encouraged to experiment.

Xen vs. KVM

So it's fair to ask: how do the hypervisors stack up against each other? Which should a data center pursue?

Three industry analysts generally agreed that while KVM is not as mature as the older Xen hypervisor, it's catching up technologically. First, Xen had already paved the way. Second, Xen required a lot more work as a complete, self-contained hypervisor than did KVM, which could simply assimilate the required components and drivers from the Linux kernel.

Third, KVM, has some advantages over Xen: It's lightweight, automatically includes the latest drivers as well as the newest Intel and AMD chipsets given that it's embedded in the Linux kernel and is a "very easy fit," said Chris Wolf, a senior analyst at analyst firm  Burton Group. "KVM has potential," and it benefits from ongoing open source projects, Wolf said.

That said, KVM does pose problems, according to Wolf and other analysts: KVM has a small share of an increasingly competitive hypervisor market (already divided among VMware, Microsoft Hyper-V and Xen), it lacks a third-party vendor ecosystem for management and support services and it has still-experimental status, with not a lot of proof yet of its performance in production.

George Weiss, a vice president of research at Gartner, who criticized Xen for its immaturity last year, said that Xen is now robust and scalable with a solid supporting environment. Xen's disadvantage is its significantly larger code base, which could translate into reliability, availability and security problems down the road, he said.

Without clear data to prove the superior performance of one hypervisor over another, the issue boils down to market issues and vendor ecosystems, Weiss said.

From the data center perspective, Xen might be a better choice for highly distributed workloads or those that need to integrate with Hyper-V or VMware, but KVM might be a better choice for a Linux-based shop seeking to scale up a single, vertical system, he said.

Can KVM step up to market challenges?

Like Citrix Systems's acquisition of Xen, Red Hat's purchase of KVM puts the formerly small startup under a larger umbrella, giving KVM a home base with a centralized development team and a champion, he said.

"It's up to Red Hat to lead the charge with its ISVs [independent software vendors] which it claims can certify to KVM readily," Weiss said. "Red Hat has to … demonstrate that they have the ecosystem and they can deliver … and can marginalize Xen as just another hypervisor."

Tony Iams, a senior analyst at Ideas International, agreed.

"If anyone can pull this off, it would be Red Hat" because of its large market presence, Iams said. "Still, [the hypervisor change] is a huge burden not just because of the baseline code changes but testing an incredibly broad set of x86 hardware."

"[The switch to KVM] is also a step back for Red Hat, because it had significant development invested in Xen, and now faces catch-up development and a ton of x86 hardware testing, he said. In addition, Red Hat must enlist the support of the top virtualization management and monitoring vendors, whose products help customers gain the hoped-for ROI, to support KVM, he said.

Red Hat also must demonstrate to customers that KVM will run on other operating systems as well as it does on RHEL and that KVM can handle I/O-intensive workloads, he added.

At Red Hat Summit 2009, Red Hat officials were upbeat about KVM and have moved forward without a backward glance at Xen.

Tim Burke, Red Hat's vice president of engineering, said that Red Hat already runs much of its own infrastructure, including mail servers and file servers, on KVM, and is working hard to promote KVM with key original equipment manufacturer partners and vendors.

And Red Hat CTO Brian Stevens pointed out in his Summit keynote that with KVM inside the Linux kernel, Red Hat customers will no longer have to choose which applications to virtualize; virtualization will be everywhere and the tools to manage applications will be the same as those used to manage virtualized guests.

Red Hat users at the Summit who were asked about the hypervisor switch were all willing to give KVM a chance.

David Pullman, a systems administrator at the National Institute of Systems and Technology, said that the organization is doing some small-scale virtualization with Xen but he is "very interested in KVM" and does not feel betrayed by Red Hat's switch in hypervisor.

"I've been around too long for that," Pullman said, laughing.

Eric Wilbanks, a systems engineer at Pearson Education, said that over the next few months, he plans to roll out a few KVM clusters and see how well they work.

"The switch to KVM will create some challenges, but this looks an opportune time to convert," Wilbanks said. "I like KVM more than Xen; it's more natural. KVM isn't there yet [maturity-wise] but it should be in the next few months."

Regardless of KVM's merits, Burton Group's Wolf said that when it comes to third-party support, KVM is last in line, an tough obstacle to overcome this late in the game.

"Citrix, Novell and Oracle are all wrapped around Xen and moving it forward," Wolf said. "It's a big industry, and it's not easy to break in and get market share by yourself. Red Hat is out there on its own pushing KVM. And it's a lonely path right now."

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