From Paris - Gare de Lyon railway station

From Monday to Friday, there are trains called TGV (Train Grande Vitesse) every 3 hours roughly, from 6:40am to 7:40pm.
Check with your travel agency for exact time.
Less frequent for weekends and holidays.
The journey lasts 3H and costs between 60,90euros and 76,20euros for 2nd class tickets 99,40euros for 1st class tickets (+ reservation, roughly 4,50euros).

To Paris - Gare de Lyon railway station

From Monday to Friday there are 3-4 TGV's per day from 5:37am to 7:16pm.

There are more TGV between Paris and Lyon and the journey lasts only 2 hours but normal trains between Grenoble and Lyon take 2 hours. In all cases, check with a travel agency or at a railway station. On TGV you must have a reservation !