At that time of night, it might be best to see if your accommodation can arrange a transfer for you – these usually work out a bit more expensive than just turning up and getting a taxi, but should hopefully still be fairly reasonable, and at least means that you won’t have to worry about taxis not wanting to make the journey into Kovalam from Thiruvananthapuram airport so early in the morning. It also means one less thing to worry about when you get off the plane – always a good idea when you’re tired after a journey! I’m not sure if it’s otherwise possible to pre-order a taxi, so either way your hotel or guesthouse should be able to give you advice on the best thing to do.