So how did Hagoromo...?

[123fire]

Bring the previous hokages?? And are they souls are he revived him

 

[Joon]

He retrieved them from the Pure World which was stated in the Manga. He only needed chakra to summon Naruto and co.

 

[SaadBoujane]

The Kages that hagoromo brought are in Chakra state, and not physical state like the edo kages. So I guess, it's some sort of summoning technique higher than Edo Tensei that only needs a portion of chakra in order to summon the desirable person.