Private ethereum blockchain is having an ethereum client running in local mode with non-default configuration of ports and connections. Otherwise it behaves and acts like a regular ethereum node.
By contract, Quorum private blockchain offers a set of services that is not available in a regular / plain vanilla ethereum blockchain. These are:
- Privacy -- in plain ethereum all txns are public. With Quorum, you have a new ability to send and receive private txns addressed to a specific node
- Permissioning -- in plain vanilla ethereum, any new node that is aware of your private blockchain can connect to it given it has a matching genesis. In Quorum, you may configure your private blockchain to only allow connections from approved nodes
- Consensus mechanisms -- in plain vanilla ethereum you are limited to the bundled consensus algos: ethash or clique. Quorum offers a set of additional consensus mechanisms: Istanbul BFT and Raft. These are designed with network speed in mind and by default can perform a lot faster than vanilla ethereum. "Faster" in context of a blockchain generally means faster block times and faster txn commit
There are many more differences, but this is a high level overview of the differences.