piggy bank vs penny bank

piggy bank and penny bank both are nouns.

Word	Noun	Adjective	Verb	Adverb
piggy bank	Yes	No	No	No
penny bank	Yes	No	No	No

As nouns, piggy bank and penny bank are synonyms defined as:

piggy bank and penny bank: a child's coin bank (often shaped like a pig)

piggy bank (noun)	penny bank (noun)
a child's coin bank (often shaped like a pig)	a child's coin bank (often shaped like a pig)