Kernel threads vs User threads

Kernel threads - each thread contends with each other and other application, may get more total time. May take advantage of multiple CPUs.
User threads - contends only with each other, and not with other apps, so may end up getting less total CPU time. User-level threads will run on a single CPU, not taking advantage of multi-CPU systems.
If there is much inter-thread communications, having user level threads may be faster because no inter-CPU communications.