Thanks a lot for your very quick reply.
I will share my experiences they might be useful.
After carefully inspecting the code I found out that marc plugin in harvester 2.3 supports MarcXML format.
The way it selects the metadataFormat for marc plugin is as follows.
Then it gets the repository supported formats.
It gets the aliases for the schema plugin (oai_marc and marcxml). Stored in schema_aliases table in database.
Then it chooses the first alias that matches repository supported format.
So, oai_marc will always be used if it is supported by repository.
But I need to use marcxml.
Edit the schema_aliases table as follows.
2 oai_marc (Edit it to marcxml)
2 marcxml (Edit it to oai_marc)