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