Is there a direct bus from xian to hukou waterfall?

Yes, there are. You can go to the Cheng Dong Coach Station (Eastern Bus Station) to take it. Two direct ones are on schedule each day, at 8:00 and 13:20.

For convenience, you can also take the bus to Yichuan at the same station. Upon arrival, it will be very easy to take a bus to the waterfall.