There are no trains between Tavira and Seville.
The train line terminates at the Portuguese/Spanish Frontier at Vila Real Sta. Antonio.
The line from Ayamonte to Huelva on the Spanish side was torn up about 15 years ago. Also there are only about 2 trains a day from Huelva to Seville.
Bus is the only way to get from Tavira to Seville.
There are currently four services daily.
You can find the timetables on this page at Cubsur's excellent website here:
http://thomasallen.home.sapo.pt/travel/001.htm