Maybe the JVM allows for more fine grained control on PC location, and allows to "go to" different places in the code without invoking a native thread. But I know that when you create a Quasar co-routine / green thread / user thread / continuation or whatever we want to call it, then it's not the same as Akka.
In Akka, if you have a "conversation" between a million actors in a circle, you can still get 1 thread to be reused, if there is no need to use more than one. But if you try to do 3 things in 100% parallel, you might find yourself with 3 threads (from the little that I know).
In quasar, you can do all that, without invoking an OS thread, I think this is the main difference.