Connecting 14 AWG to 12 AWG for oven operation
A 30A connection will require a much heavier wiring connection than either 12 AWG or 14 AWG. It is common that a 40A or 50A cord is used to hook up a range unit. These can be purchased in several lengths with molded on plugs at the end. 

As always make sure that your outlet, wiring back to the entrance panel and the circuit breakers are sized properly for the load. The circuit components are normally rated a bit higher than the maximum current draw of the range. So a range unit that pulls 30 or 32A should be on a 40A circuit.