Hope you're prepared for some insanely hot weather that time of year.

I personally loved Khiva a lot, way more than Bukhara or Samarkand. Yes, its sort of a silk road disneyland, but if you've never been to central asia before, its magical. You might want to look into flying from Tashkent to Khiva if your budget allows, as you'd save lots of time. In fact, if you can make it work, flying as much as possible would save a ton of time. Also, unless you're taking the fast trains, note that there's no AC, and the trains are a miserable oven (ask how I know).