What is the word "knock" an example of in this excerpt from "Batter my heart, three-person'd God" (Holy Sonnet 14): volta, simile, onomatopoeia, or personification? Batter my heart, three-person'd God, for you As yet but knock, breathe, shine, and seek to mend

The first two lines of John Donne's poem "Holy Sonnet 14" express the speaker's desire that God figuratively beat ("batter") his heart so that he can be a better person and a better Christian. The word "knock" in line 2 could be said to serve as an example of onomatopoeia. This is because the sound made when the reader pronounces the word "knock" replicates the sound of knocking.
You could also say the word "knock" is an example of figurative language or is metaphorical. The speaker is asking God in those first two lines to "Batter," "knock," "shine," and "seek to mend" his heart. None of these verbs is literal; the speaker is using the terms to suggest that he wants God to help him improve as a person.
This figurative image of God taking the speaker's heart and hitting it, whipping it into shape, and shining it to perfection implies that the task of improvement as well as the speaker's submissiveness to God's will. He is turning himself over to God to allow the deity to do what he must in order to make the speaker an ideal follower of Him.
Some of the other terms in your question should also be discussed as a way to eliminate them from consideration. A simile is a comparison between two dissimilar things using the word "like" or the word "as." There is an instance of the word "as" in the line, but it does not seem to be used as a comparative in the way a simile uses "as." There is an example of a simile later in the poem when the speaker exclaims:

I, like an usurp'd town to another due,
Labor to admit you, but oh, to no end

Here, the speaker compares himself to "a usurp'd town" using the word "like," which makes this example a simile.

A volta is a turn in a poem—specifically in a sonnet. While Donne's poem is a sonnet, this word "knock" occurs in line 2, which is too early in the poem for the volta. Typically, the volta in a sonnet occurs at either line 9 or line 13 (in a Petrarchan or a Shakespearean sonnet, respectively).

Finally, personification is a figure of speech in which human traits are given to non-human entities. While it could be argued that God is a non-human entity, since we often imagine God as like humans (and refer to this entity with human pronouns) I would say that, strictly speaking, this is not personification.

The word "knock" is most likely an example of onomatopoeia or figurative language/metaphor based on its use in the second line of Donne's "Holy Sonnet 14."