There are two situations where running applications directly on the hypervisor might be beneficial though:

- If you have to run multiple untrusted (and possibly malicious) tasks on same host. In this case virtualization provides better isolation/security than just containers or processes (which would be vulnerable to kernel exploits, or other ways of gaining root inside a container allows to take control of the host)

- If the hypervisor is fixed and you can't touch it (i.e. Amazon EC2). In this case running applications directly on the hypervisor might eliminate some of the virtualization overhead.

The attack surface of a VM is rather large, particularly when you consider the dozens of emulated devices and instructions. For example, there was a notable hole in SCSI emulation recently discovered which let any guest overwrite any part of a host's disk (by sending some obscure SCSI commands which weren't being filtered by the virtualized device properly).

Wait, you're saying Google NaCl could be going the way of Java -- being used for server side containers, rather than client side apps? (I don't really see why you'd want that over using a single process(tree)/binary compiled from go...).

[1] https://news.ycombinator.com/item?id=3746222 [2] http://zerovm.org