Severin Safari Camp - is situated in Tsavo West, and we actually spent a night here on our honeymoon - where for an additional $50 they upgraded us to a suite, which was wonderful. The camp is not fenced and animals can walk through - and often do, particularly hippo - with which we had a fairly close encounter!! However, do not worry, you are always escorted to your tent by an armed Masai guard. The camp also has excellent environmental credentials, recycling water, state of the art efficient generators with heat exchange systems, etc.
It also has a fabulous spa.- see link to their official website here