Dear aspirant, 

Yeah.Nativity certificate is necessary for every candidate even candidates inside kerala for proving their nationality.It is to be produced before the entrance commissioner and is to be uploaded at the time of keam registration.For proving nationality any of the given below can be used.

Birth certificate

SSLC certificate(It can be used only if your place of birth is there in it)

OR digitally signed Nativity certificate from Akshaya

Hope this will help you. 

Good luck