Beijing and Xi'an almost has the same weather during every May. Take Xi'an for example, the whole city is becoming hotter and hotter during these weeks. And, when enter into May, short-sleeves T-shirt, skirt and sandal is the most comfortable clothes then. It is also somewhat dry with little rainfall/ Shanghai may be a little hotter than Beijing and Xi'an, but this city is much moister and has occasional rainfall during daytime. Guilin should be cool thank for its good natural environment~ In a word, take your summer wears, sunblock, sunglass as well as a thin long-sleeves T-shirt and umbrella.

Nice trip!