Green Threads implement Userlevel Threads. (See Linux).
Disadvantages is that SMP (Symmetric Multiple Processor) is not supported and that the efficiency could suffer under this approach.
Native Threads
Native threads get rid of the disadvantages of Green Threads. The only disadvantage is that all operations become kernel-operations, which might cost time. IBM JDK supports only Native Threads. You will need a Linux 2.2 kernel in order to run native Threads.
Java Threads in Linux
Due to soem issues in the Linux Kernel (namely the kernel does not allow several kernel threads to be packed inside a process), each Java thread is implemented by a corresponding user process whose process id and status can be display using the ps-command. [This is certainly true for 2.3 and lower kernels. Not sure about 2.4 kernel.]
