Yes, every emf may be a potential difference
      Emf E of a battery is equal to potential difference V when circuit is open or when internal resistance of battery is zero

      Since V= E-Ir            in open circuit I=0   or without internal resistance r=0
                                       	In both cases            V=E
	Hence emf become potential difference.

No, every potential difference is not an emf because potential difference is taken across resistor or component in a circuit when circuit is closed and current is being supplied by battery. 
And in that case V always less than E due the effect of internal resistance.

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