I don't think Maw of Malmortius is good item for Xin. It only gives 40 mr and no HP also the passive is hard to utilize.
I think most of the time Mercury's Treads and Wit's End is enough against an AP threat. However, if you are against heavy AP comp, or if you are behind and need more tankiness instead of Wit's End, you can buy Kaenic Rookern (vs AP Burst), Force of Nature (vs AP Burn damage), or Spirit Visage (if your team has peelers).
Also I think, Sterak's Gage is a must have item for Xin