The differences between Performance max Retail and Standard Shopping campaigns

A PMax Retail campaign has only one AssetGroup which means that you can only have one Product Group tree per campaign since a tree is always linked to one AssetGroup. A Standard Shopping campaign supports multiple AdGroups.
A PMax Retail campaign does not have Negative Keywords.
Bids management is done entirely by Google while you only get to set the budget.
A PMax Retail campaign uses only the Maximize Conversion Value with an optional Target ROAS as a bidding strategy.