We are a group of 3 friends from india who want to travel to spain in december.
We are planning a roadtrip. Is it advisable to travel in december?
How will the weather be in barcelona and andalucia? Which car company is the best for a roadtrip. Are hostels better during this time or should we book an apartment? Should we make reservations prior to our arrival or we can go there and get good deals? If we go there without booking will the hostels be booked completely? How many days would be enough if we start from barcelona cover valencia granada seville cordoba and end in madrid?

Have you actually done any research? Whatever you do will depend on your budget and interest. Spain in December celebrates the season and it will be busy everywhere, so you will need to make reservations if you want to spend the night somewhere.

Are you aware what the weather will be at that time of year?

And how many weeks do you have for this trip?

It would also help to know your interests and your nightly budget to help with accommodation.

Adding to what other people said, travelling by car from Barcelona to Valencia can be 3 hours.
From Valencia to Granada, Seville or Cordoba the roads are not good and is almost better to go through Madrid.

Take the high speed trains and check weather websites with past years's weather statistics to have a complete idea of what it will be like that time of year.
And Spain will be dilled with Xhristmas teasitions which is something you may or may not enjoy.