MemoryBlockStore keeps StoredBlocks in memory and it is used primarily for unit testing.

Whereas SPVBlockStore holds a limited number of block headers in a memory mapped ring buffer. With such a store, you may not be able to process very deep re-orgs and could be disconnected from the chain (requiring a replay), but as they are virtually unheard of this is not a significant risk. You can find the reference document from the following link:

MemoryBlockStore https://bitcoinj.github.io/javadoc/0.12/org/bitcoinj/store/MemoryBlockStore.html

SPVBlockStore https://bitcoinj.github.io/javadoc/0.12/org/bitcoinj/store/SPVBlockStore.html