Nano uses 'Open Representative Voting' and not PoS. Here are the differences:

When Nano is mentioned, the coin is normally lumped in with the PoS crowd. This is indicative of the thought on this sub, held by probably the majority of people, that coins either use PoW or PoS as their consensus mechanism. However, I want to show you how 'Open Representative Voting' (ORV) is actually another option that is distinct from PoS, although admittedly similar.

Disclaimer: Although I own Nano, I'm not arguing for ORV over PoS, I'm just showing how the two are different.

From the Nano documentation website, the differences between ORV and PoS are:

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• There is not one monolithic blockchain that requires leader selection (i.e. a staker or a miner) to extend
• Representatives do not create or produce shared blocks (groups of transactions)
• Each Nano account has its own blockchain that only the owner can modify (representatives can only modify their own blockchain)
• In Nano, a block is a single transaction (not a group of transactions). Transactions are evaluated individually and asynchronously
• Users can remotely re-delegate their voting weight to anyone at any time
• Anyone can be a representative
• No funds are staked or locked up
• Representatives do not earn transaction fees
• Representatives cannot reverse transactions that nodes have locally confirmed (due to block cementing).

Tl;dr Nano uses ORV and not PoS. They are very similar, but still distinct, consensus mechanisms.