I had recently started poking around the lguest hypervisor. From my limited reading I believe 2 of the 3 memory subscription choices mentioned in the article are present in Linux. Existing linux based open source hypervisors like kvm etc use paging/swap mechanism (i,e, for x86 - the paravirt mechanism). Ballooning is possible using the virto_balloon. Kernel shared memory in linux allows dynamic sharing of memory pages between proceses - this probably doesn't apply to virtualization.
I couldn't find any CPU over-subscription thing in open-source hypervisors. It seems to be the only area where open-source hypervisors are lacking.
On an other note, established players like IBM tend to use Type-1 hypervisors (link) for enterprise servers, it would be interesting to see how this company fares against them.
Possessions increase to fill the space available for their storage. -- Ryan