I hate this two faces comments. In an article in a blog of the Wall Street Journal about xVM, an analyst is cited:

Al Gillen, an analyst at IDC, questions whether many customers really have the expertise to modify their virtualization code. “It sounds good to say it’s open-source and people can look at it, but in reality most customers aren’t capable of doing that anyway,” he says.

This is two-faced … whereas the analysts told us in the past, that an operating system must be open-source to enable you to fix bugs and to improve code, this analysts thinks that companies don´t have the expertise to modify hypervisors. While i think this is correct for hypervisors, i´m strongly opinionated, that this is valid for an operating system as well. And when you look at the code size of an operating system and a hypervisor you will notice, that a operating system is vastly more complex than a hypervisor. There are not many people who can really modify the scheduler or the virtual memory management in a meaningful way. Thus open source is important for both components or it´s unimportant for both …