Linking verbs link the subject to another word that renames or describes the subject.
Examples of linking verbs are: be, become, seem, smell.
For instance, in the sentence Maria will become a doctor, the subject, Maria, is linked to the word doctor which describes her.
Helping verbs are verbs which accompany the main verb and help to either form an interrogative or negative form, or they help in the formation of a tense. Examples of helping verbs are: would, will, can, could.
Action verbs are verbs which describe an action, they describe what the subject is doing, such as run, eat, read.
(Pronoun verbs do not exist as a category of verbs.)