Hey, Mr. Dorothy Dar.
Good idea to take a train to Shenzhen from Hongkong and then go on to Guangzhou also by train. These train tickets are easy to buy. While from Guangzhou to Guilin, it is not very easy to get the ticket if you still want to take train. Now in China it is the summer vacation, many students go to travel outside. Train tickets at this time are always hard to purchase. It all depends on your luck. There are only two trains from Guangzhou to Guilin, but there are about ten flights everyday. Surely train is much cheaper. But if your time is fixed, you'd better choose to fly. The situation is the same for the journey from Guilin to Xian. Now it is your time to think about it.