We are 3 persons travelling to China for the first time. Following is the tentative itinerary.
03/11/11 Kolkata-Kuala Lumpur 1 night stay and then on 05/11/11 KL to Tianjin reaching at 2.35 pm, further travel to Beijing by train/bus.
5th, 6th, 7th, 8th Stay at Beijing. (are 3 days enough?)
8th or 9th Travel to Shanghai by train.
Shanghai - How many days should we see to explore the city? Is one day enough?
Then travel to Hangzhou by bus or train.
Hangzhou - We wish to spend 1 day here.
Hangzhou - Hongkong : Please suggest mode of transport, how and when can we travel.
We want to stay in Kowloon area in Hongkong, and spend 2 days in Hongkong and 1 day in Macau.
Travelling back from Shenzhen to KL on 16th Nov. 21.40 flight., and then connecting flight to India. All flights are booked on Air-Asia.

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Hangzhou to HK should be by plane. And when you are in HK, just spend the 3 nights there and do a day trip to Macau which is only an hour away by ferry.

While you are in Kowloon, you really should try to go out to Lantau Island to Big Buddha. I was pleasantly surprised to find the aerial scenic cable car ride (about 30 minutes each way) to Big Buddha from the end of the subway line. I can't recall the name of the 'town' that is there (with hi-rise apartment buildings, a shopping mall, eateries, etc. and the cable car ride. You will pass Hong Kong Disney to get there. I thought it was well worth the time, and I was just there in April of this year.

Either fly directly from Hangzhou (HGH) to Hong Kong (HKG), or you may save some money by flying to Shenzhen (SZX) and transfer to HK.

Alternative for those with lots of time, which you you don't, is the direct train from Shanghai. Runs every other day and takes about 20 hours. Or train from Hangzhou/Shanghai to Shenzhen/Guangzhou, and connect to HK.

Thank you all for this information. We have booked hotels in Beijing and HK. We will buy train ticket on arrival in Beijing for Beijing to Shanghai.
Is it possible to do a day tour of Hangzou by staying in Shanghai, or would you suggest travel to Hangzou and then further fly from there? Which is budget airline from Hangzou to HK/Shenzhen?

Hangzhou is only an hour by high-speed train from Shanghai, and very frequent, so you can definitely do a day trip. Either way is fine.

All Chinese airlines are similar, whether they're called low cost or not.