There are direct flights from xian to Jiuzhaigou in high seasons. You may check the cost and duration time at China Flights(https://www.travelchinaguide.com/china-flights/)

No direct train frm xian to Jiuzhaigou, you need to go to Chengdu first(about 16 hours;the hard seat is about rmb115, the hard sleeper is about rmb200, soft sleeper is about rmb300) and then change to a bus to Jiuzhaigou.