Her parents are threatening to hit her if she covers her face

Know that wearing hijaab is a duty for women, and it is not optional. Allaah says (interpretation of the meaning):

and to draw their veils all over Juyubihinna (i.e., their bodies, faces, necks and bosoms, etc.) [al-Noor 24:31]

and:

and do not display yourselves like that of the times of ignorance [al-Ahzaab 33:33]

Know that the complete hijaab which you are observing is something which is very good, so you should thank Allaah for it and ask Him to make you steadfast. Be steadfast and Allaah will strengthen your heart.

What your mother has told you about hijaab meaning to cover the hair only is not correct. A womans beauty is in her face, not only in her hair, and Allaah has commanded that her beauty should be covered, as He says (interpretation of the meaning):

and not to reveal their adornment [beauty] [al-Noor 24:31]. The focal point of beauty is the face, so it must be covered, according to the most correct view among the scholars.

Know that Allaah is testing you by means of your mother, so see whether you will obey Him or her.

You have to advise your mother in a number of ways, including the following:

Speaking to her directly, or giving her a tape by one of the scholars which clearly outlines the ruling on hijaab, or giving her a book on this topic. Or you could call one of the shaykhs on the phone and ask your mother to listen to his answer; or take your mother to an Islamic lecture where the correct ruling on this issue will be discussed or some other means.

If you are patient, keep hammering the point and pray to Allaah to open her heart at the times when duaas are answered, you will get results in sha Allaah. If your mother does not respond, then you do not have to obey her in this particular matter, because the Prophet (peace and blessings of Allaah be upon him) said: Obedience is only in matters that are right and good. And there is no obedience to any created being if it involves disobedience to the Creator. At the same time, you have to be kind and respectful towards her, because Allaah says (interpretation of the meaning):

As for those who strive hard in Us (Our Cause), We will surely guide them to Our Paths (i.e., Allaahs Religion Islamic Monotheism). And verily, Allaah is with the Muhsinoon (good doers). [alAnkaboot 29:69]

and:

O you who believe! Endure and be more patient (than your enemy), and guard your territory by stationing army units permanently at the places from which the enemy can attack you, and fear Allaah, so that you may be successful. [Aal Imraan 3:200].

And Allaah is the True Guide, there is no god besides Him.