To be more specific: the current active fleet of hypervisors (xen, vmware..) is much younger on average than the current active fleet of traditional operating systems (bsd/linux/solaris).
By "younger" I mean the source code was written, and the interfaces defined, much more recently.
Younger than what? VM dates to the first half of the 1960s.
The concept of the OS is older, but most OSes in active use now aren't.