If you are trying to find out what the right hypervisor is for your private cloud or IaaS then you might be asking the wrong question…
Do most applications really need an OS and hypervisor is a better question?
One company of the companies that is exploring this area is Joyent. Thier SmartOS is like the mix between a virtual machine and a combined OS + hypervisor. Instead of installing a hypervisor, on top an operating system, on top an application server or database, the Joyent team thought it would be more efficient to try to remove as many layers as possible between the application/data and the hardware.
According to publicly available videos and material, their SmartOS is based on a telecom technology for high-scalable low-latency application operations. Unfortunately Google does not seem to be able to answer which telecom technology it is. So if you know the answer, please leave a comment.
The idea of running applications as close to the hardware as possible and being able to scale an application over multiple servers is the ultimate goal of many cloud architects. Joyent claims that their SmartOS runs directly on the hardware. On top of SmartOS you are able to install virtualization but ideally you run applications and data stores directly.
The next step would be to combine the operating system with the virtual machine/application server or database server into one. Removing more layers will greatly improve performance as can be seen by Joyent’s performance tests.
So the real question is: do we need so many extra layers?
A distributed storage system, a virtualized webserver, a virtualized app server, a distributed SQL-accessble database or NoSQL solution that would run straight on hardware with a minimal extension to distribute load over multiple machines would be the ideal IaaS/PaaS architecture. It would give customers what they really need: performance, scalability, low-latency, etc. Why add a large set of OS and hypervisor functions that at the end are not strictly necessary?