1

From Xian to Jiuzhaigou, consider going via Guangyuan and Jiangyou; see http://www.jiuzhai.com/language/english/visit_travel.html . Or fly direct from Xian if there are direct flights.

Is Hainan for the beach? It's ok, but not that good. I'd probably spend longer in Sichuan rather than the 5 days on Hainan - Emeishan and Leshan for example, or the small town of Langzhong, or longer around Yangshuo / Guilin (see the rice terraces).

If you can, try to arrange your flight home from Hong Kong. Can you book an open jaw - fly into Beijing, and fly out of Hong Kong?

2

You're going to have a big problem in first week of August getting sleeper train tickets. Probably already sold out.

1) Strongly suggest you fly directly from Xi'an to Jiuzhaigou, money well spent. The airport at JZH is closer to Huanglong, so stay in staging town Chuanzhusi on the afternoon of the 3rd, do Huanglong on the 4th and late aft the 4th move to Pengfeng village close to Jiuzhaigou park entrance. 5th and 6th at Jiuzhaigou. Good luck trying to find accommodation at reasonable price (or any price). Aug 7th take bus to Chengdu but allow 10 hours due to construction, you might get lucky and make it in 8 hours. Aug 8th-11th Chengdu area. I definitely agree with drumbrake to spend longer in this region.

Aug 12 fly to Guilin and get to Yangshuo immediately. No need to hang around Guilin.
Aug 13-16 in Yangshuo and Rice Terrace areas. You can do the best parts of the Li River from Yangshuo and Xingping.
Aug 17 fly Guilin to Hainan (may have to connect in Guangzhou or Shenzhen). Personally, I'm not impressed with Hainan and this time of year is very rainy (typhoon season) if you are expecting a sunny beach. Aug 18-20 in Hainan.
Aug 21 fly back to Guangzhou or Shenzhen, border cross to HK.

Notes:

--If you go per your original itinerary from China to Hong Kong then back to China (Hainan), you will need a double-entry visa.
--Flights to/from Hainan are generally pretty expensive and especially from HKG.
--An open jaws return ticket from HKG is much better than going back to Beijing to catch a flight home
--If you do have to go back to Beijing from Hong Kong by flight, and have used up all the entries on your Chinese visa, you are allowed a transit stop of up to 24 hours with no visa in Beijing before your international flight out. You can leave the airport during this time. Mind your timing very carefully.
--If you have a multiple-entry visa for China, no worries at all. But those aren't easy to get for most nationalities.
--Heed our warnings about Hainan--probably the most useless place on your itinerary.
--And I give you fair warning that Jiuzhaigou is now packed out, and if you haven't already made reservations somewhere, you may have a very tough or very expensive time of it.

3

I just got back (still jet lagged) from 5 incredible weeks in China. I agree with the previous writer on a couple items. You can fly into Beijing (via Hong Kong) and home direct from Hong Kong - I did it on Cathay Pacific (good legroom too). Yes, after you fly into Guilin leave immediately from airport for Dragon's Backbone rice terraces (by bus or splurge for private shuttle)- I spent 2 nites at Wisdom Inn in Dazhai so spent full day hiking. Then go direct to Yangshuo (again, I splurged for driver - as a solo woman traveler its worth it to me). I spent 6 nites there and its now one of my favorite places on earth. Once there you have many options for boat rides on Li river and real bamboo rafts on smaller /even better Yulong River. The boat ride all the books mention from Guilin is on a huge steamer size boat,same glorious views but much more fun on smallermotorized "bamboo" boats where everyone plays with giant water guns - great fun as its incredibly hot/humid. Also you might want 2 days in Xi'an - one for day trip to terracotta warriors (by bus/no tour nec) and one for city (bike the walls) - the city itself is not too endearing best stellar sights.I loved China so much that whatever you do it'll be great!

4

Leave out Hainan, save more times for mainland China himself, it also save your money from flight to Hainan from HK.
Shorter Guilin, more times for the surrounding of chengdu and Beijing, or add Yunnan province in your schedule.

But China is huge, I spent 2 week for Yunnan province and Sichun province respectively, and visited the most major tourist site only.