Hi, i don't think that there is any resort on Kefalonia which can be described as "lively" There is certainly nothing on the island that can compare with Malia, Tsilivi or the like.
Kefalonia does have bars which will stay open as long as you want to stay there and yes there will be music playing and also the odd disco/ night club but it is quite low key.
The two resorts will probably appeal to you most would be Skala or Lassi. Both have plenty of good restaurants and excellent beaches. The beach at Lassi is probably the better one with fine sand whereas Skala is a bit more coarse with lots of shingle in it.
Lassi is also close to the capital Argostoli where there are even more clubs and bars. You can walk to Argostoli from Lassi in about 30 minutes or easily get a taxi.
Skala is at the South east coner of the island and the transfer from the airport usually takes about one and a half hours. There is one night club/ diso in the resort at the Scandanavia bar / restaurant. this is open every night. There is also the "Spilio" night club half way between Skala and Poros and is open every weekend.This is a very popular place with the islanders.

John