1hr drive from either Alicante or Valencia airports. You can either hire a car or take a taxi directly at the airport.
If you are hiring a car, I suggest to book it online prior to the trip to ensure a good deal and try to avoid queues at the airport.
For approx 10€ you can also take a bus transfer to Alicante town or Benidorm from the airport and then a direct bus to Denia. This will take over 2hrs but will be much more economical.