A SPV (Simplified Payment Verification) server is a type of server that allows users to verify transactions on a blockchain without needing to download the entire blockchain. This is particularly useful for verifying transactions on Bitcoin Cash (BCH), Bitcoin (BTC), and Litecoin (LTC). In a SPV server, the client (such as a wallet or a user) can request a proof of a specific transaction from the server, which then provides the client with the necessary information to verify the transaction without needing to download the entire blockchain. This is done by using a technique called Merkle tree proof, which allows the client to verify the existence of a transaction in the blockchain without having to download the entire blockchain. Fulcrum is an example of a SPV server that supports BCH, BTC, and LTC. It is designed to be fast and nimble, and is a drop-in replacement for ElectronX/ElectrumX servers.