From Mexico City to Xalapa

To travel from Mexico City to Xalapa there are these options:

Bus

If you wanna travel directly from Mexico City Airport to Xalapa, there is a van that travels three times per day to Xalapa and the trip takes around 5 hours. The trip is not so comfortable however and there is no toilet in the van.

Another option by bus is to take a taxi* from Mexico City Airport  to Mexico City Bus Station "TAPO". There are buses leaving to Xalapa almost every hour and there are 3 types: 
        -Economic (ADO normal)
        -Middle class (ADO GL) 
        -First class (Platino)

Adam and I have been taking "the economic option" and we were satisfied.

*For security reasons if you take a taxi in the airport make sure that you take one of those that have service points in the airport area. You pay for your taxi at the service point and you get a ticket, so you don't pay the driver in the taxi. Simply show your ticket to the driver.

Airplane

You can take a flight from Mexico City to Veracruz. Unfortunately, there is no airport in Xalapa. Therefore you have to take a bus from Veracruz to Xalapa.
There are three airlines flying from Mexico City Airport to Veracruz: Interjet, Volaris and Aeromar.
These option is obviously more expensive than the pure bus options above. In general you don't even save time.