Xen and KVM, like other virtualization tools, can be used as part of a comprehensive datacenter architecture. One of the most important value propositions offered by both of these tools is low initial cost (read "free"). Depending upon the implementation (Citrix, Red Hat, SUSE, Virtual Iron, Oracle, Sun, Qumranet, etc.) these tools may offer outstanding performance. Depending upon the expertise level found in the organization's IT department, they may not be the most cost-effective tools.
A key challenge faced by any open source project is getting mindshare. It's a truism that if decision makers don't know about a product, they won't consider it. If they don't consider it, they'll select other approaches. There are too many people shouting out their own Xen messages. Comprehension appears to be lost in all of the noise. Qumranet and, now, Red Hat are tooting the KVM horn. If we look at VMware or Microsoft, there is only one company, a company with deep pockets, talking about their own technology.
Better funded (from a marketing perspective) competitors, such as VMware and Microsoft, seem to get the headlines. This can be directly associated with mindshare.
I, for example, get messages and calls from the folks at VMware and Microsoft on some topic or another every week. Citrix and Virtual Iron speak with me about once every couple of months. Red Hat, Sun and SUSE speak with me about once every couple of quarters. Oracle speaks with me once in a blue moon. If that level of communication is similar to that received by other analysts, journalists and consultants, the Xen message doesn't stay on the radar screen for long before it is drowned out by everything else happening in the industry.
I've actually worked with several of these players in the attempt to help them get their messages out more effectively in my role as Principal Analyst of the Kusnetzky Group. In my role as blogger for ZDnet or columnist for Virtual Strategy magazine, I present information on these folks when they do something worthy of comment.
Why do you think that Xen and KVM aren't "top of mind" for more business decision makers?