are there night tains (fast or regular) in following order Guangzhou, Xian, Beijing,Shanghai,Hongkon

First I think it's nearly impossible to see the 5 cities in 8 days, as each of them has a lot of interesting places. For example, you'll need at least 3 full days in beijing, 2 full days in xian, and 2 or 3 days in shanghai, etc.

For the trains:
Guangzhou -Xian: 22 hours, means a whole day and night;
Xian-Beijing: night ones take about 11 hours;
Beijing-Shanghai:night ones take over 11 hours;
Shanghai-Hongkong: over 18 hours.