Typically, a virtualized server runs virtual machines (VMs) with a one app/one guest OS combination, and VMware - being interested in the total cost of ownership per virtual machine - would like to reduce the footprint of each VM so as to cram more in to each physical server.
Cavalliere says, VMware's ESX hypervisor can detect memory pages that are duplicated by multiple instances of guest OSs. For example, if there are several Windows XP/application VMs running then ESX won't load each XP instance's full set of memory pages as some single pages can be shared between the multiple XP instances.
What would be good would be for the multi-tasking part of each guest OS to be simply discarded. It's not needed and, as VMware says, a paravirtualized guest OS (one that knows it's running under a hypervisor) could ignore it. The guest OS running under a hypervisor could be stripped down to a kernel, letting the hypervisor handle things like multi-tasking and talking through drivers to physical resources.