That question was debated here at a few weeks ago i think.
Anyway, the short answer from someone not into such subjects, i think that a Zhing Kham refers to a given place where a Buddha arises and teaches. In this way you may have a impure Zhing Kham, like Earth, a place born from Samsara and Kamma, so to say. Our Earth is the place where Buddha Shakyamuni taught and can only appear as pure to the eyes of very pure people (biddhistavas, enlightened people, etc), otherwise will seem as impure to the eyes of "ordinary" people.
But we also have pure Zing Khams, created by Buddhas. Such Zhing Khams are pure, even to the eyes of ordinary people; also, its not easy to reborn their. The Pure Land of Amitabha and the Pure Land of Medicine Buddha are 2 examples of pure Zhing Khams.
Please, note that i may be wrong. I am not a schoolar and i have heard about Zhing Khams only here at a few weeks ago! If i am wrong, please, someone correct me.