What is Green Thread.

wiki http://en.wikipedia.org/wiki/Green_threads

The Wikipedia article Green Threads explains it very well. 

In a nutshell, Green threads are "user-level threads". They are scheduled by an "ordinary" user-level process, not by the kernel. So they can be used to simulate multi-threading on platforms that don‘t provide that capability.

JVM

Green threads were abandoned in the Sun JVM for Linux as of the release of version 1.3 (see Java[tm] Technology on the Linux Platform). That dates back to 2000.

Erlang VM

Erlang doesn‘t use "Green Threads" but rather something closer to "Green Processes": processes do not share data memory directly but do so by "copying" it (i.e. having independent copies of the source data).

Reference 

http://en.wikipedia.org/wiki/Green_threads

http://stackoverflow.com/questions/1947180/whats-the-difference-between-green-threads-and-erlangs-processes

Green Thread