Fly/train from Paris to Sevilla?

The simplest train route takes over 17 hours. You can fly non-stop on a budget airline for around $100 US. See the search engine skyscanner.com.

Fly by all means if possible.

This comes down to the old "Why are you going?" question. If you are going to see Paris and Seville or have time constraints you should fly. If you are going to France and Spain and have the time you should take the train. You see very much more from the ground than you see from 30,000 feet.

Although I like traveling by train within Europe, that is a long journey. I would fly but only you know how much time you have and what sort of travel you like. You may, or may not, see a great deal from the train, I am not sure but on the fast trains the scenery flashes by. You will see a lot more in Paris and Seville.

Fly. This route are too long for a train trip. And likely more expensive even in the "simplest" combination of 2 high-speed trains. No point in using trains for such trip unless you are a train geek.

The only reason for training would be to make multiple intermediate stops. If you're just going from one to the other, fly.