The Account Manager's job to keep the client happy and in some cases, expand and upsell. Meanwhile, the Business Development Manager establishes new relationships for the Account Manager to build upon
Typically BDM do not get an account base to work from. You start from scratch and find all net new customers.
The Hunter or BDM will expect to be paid differently than a farmer or account manager. BDMs will usually get paid a higher salary and higher bonuses. This is due to their capabilities and also their capabilities to generate business from nothing and deliver a result usually with a higher value on profit and margin.