Towels: If you have chosen the option of hostels our recommendation is that you bring your own towel, as the vast majority do not offer this service or will only give you the option of renting one. But if you stay in private rooms in hostels, hotels, rural houses etc. you will have towels in the room as well as a small courtesy kit with shampoo and shower gel.
Hairdryer: The vast majority of hostels, hotels and rural houses have a hairdryer in the room. In hostels the answer is not so certain, they may not have it or you may have to ask for it at reception. However, you can always take a small travel hairdryer with you, or ask a hostel mate to bring one. The Camino is also about sharing!