Hello Nikunj,

regarding your Question about the Datev export, Odoo offers a standard Datev Export. You can find it by going Accounting => Reporting => General Ledger => Choose the Customer Invoices in Journals =>Correct date and click on the export Button. A zip file will be downloaded. You can give this to the your tax Consultant. 

Remember: You have to create creditor and debitor accounts manually in the contacts before you sending this document.

There are also different Odoo partners in Germany, who have Datev Export modules that can help you that too.