They are very similar, but your answer hits on a good distinction between them. There was a lot of work in Exokernels to allow them to run multiple libOSs on th

They are very similar, but your answer hits on a good distinction between them. There was a lot of work in Exokernels to allow them to run multiple libOSs on the same hardware safely and securely. Another distinction was that Exokernels did require a single language. For example, each OS asked for packets using a network interface language. I guess a way to look at it is alluded in another comment I just saw here. Virtual machine hardware was not even a consideration in those days. An exokernel played the role of VMM and libOS. The unikernel technology of today pretty much requires a VMM layer (which is fine).