MODVAT was a scheme where the manufacturer had option to take input credit on their duty paid purchases, for duty liability on their manufactured goods,
as the bill of VAT got introduced in 2003, which replaced the state sales tax, so the confussion had to arrive between vat and modvat as the vat is total state taxes and modvat is related to central excise only.
to avoid the confussion the excise duty was renamed as "CENVAT" (Central Value Added Tax), and Cenvat Credit replaced the theory of Modvat of excise duty ( now cenvat duty) input credit available to set off the Cenvat duty liability of dutiable manufactured goods.
Cenvat scheme extended the credit of service tax, capital goods available for input credit for payment of duty towards taxable manufactured goods / taxable services.