Cloudstack + XenServer is one of the best solutions in today's scenario considering the fact that both Cloudstack and Xenserver are Opensource now. They have a huge community working towards further development.
Main point to be noted is that Xenserver now has all advanced features that anyone could think of such as live migration, Storage migration, Dynamic Memory Allocation etc., etc., Cloudstack 4.2, the latest version is a big step towards the future of Cloud Computing Industry - IaaS. We have a great deal of experience in both the domains.
AssistanZ - Beyond Boundaries... Cloudstack Consultancy / 24x7 Web Hosting Support / 24x7 Server Management / Infrastructure Management Services Web & Mobile Apps Development / Web Designing Services / Php, Grails, Java Development