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.
The Wright Bothers weren't the first to fly. They were just the first not to crash.