You are not required to use do in this context.
It is sufficient to write:
Dearly I love you
as an alternative to I love you dearly.
But there are many contexts - often poetic, philosophical or reflective - in which we insert the auxiliary after the introductory adverb to create a certain mood:
Painfully do we learn the lessons of life
Wearily does he make his way home.
Quietly does she sleep in her bed.
However, Dearly love I you is not grammatical. You need to write either:
Dearly do I love you
I do love you dearly.
Note that placing the adverb dearly at the end of the sentence creates a different nuance, as though someone were insisting on his/her love in the face of doubts.