How do I show 20% tax at checkout page with product original price? Means we not adding extra 20% to the product price but we should show 20% including tax
If I understand you correctly, this should solve your problem:
Navigate to Admin > Settings > Configuration > Sales > Tax and in the tab "Calculation Settings" set "Catalog Prices" prices to "Include Tax".
This sets whether catalog prices entered from Magento Admin include tax.
Also, in the same configuration section (Admin > Settings > Configuration > Sales > Tax), you will find the display setting, so you can adjust that to fit your needs.