Druids from Azeroth come to Outland and in a few, rare places manage to re-grow healthy foliage in the sickened soil. These realms are lush and verdant, a window into Draenor’s past when it was a thriving, green world. Travelers speak of unique creatures in these areas, magically evolved to adapt to the new environment.
In The Frozen Throne, the night elves were growing bases in Hellfire Peninsula, and Cenarion Expedition members can be found in several different zones in The Burning Crusade. Indeed some of the quests in Blade's Edge Mountains confirm that Cenarion Expedition had magically grown some of the patches of forest in Blade's Edge Mountains to replace some of the forests that once grew there, which were destroyed by the Burning Legion, or by other causes.
The term Green Havens seems to encompass more than one location in the central landmass of Outland, so it may include the well-foliaged zones seen in Outland in the Burning Crusade, such as Terokkar Forest, and/or parts of the Blade's Edge Mountains. Nagrand was one of the last areas of Draenor that was left largely unaffected by the corruption of the world (although it wasn't completely unaffected). Due to the fact that it was mostly unaffected it is unlikely that it is one of the Green Havens.