Thanks again for your answer.

If I am not mistaken there are two adverbial forms of the verb читать as follows;

1- читая >> present tense
2-читавши>> past tense.

and for the verb прочитать as follows

1-прочитав >> represent the past tense.
2-прочитавши >> represent the past tense.

So here what is the difference between 1st and 2nd adverbial forms of both verbs? 2nd forms are used often?

I think this is a different concept than читать - читая -читав (which does not exist according to the academic grammer)

Or читавши and читав mean the same?