A model of computation vs an abstract machine
An abstract machine typically defines instructions with opcodes and operands. A Warren abstract machine, a java virtual machine, a Turing machine.  A programming model defines operations at a higher level, and could be independently supported by a number of different machines.  For example the actor programming  model says that an actor can send a message, create a new actor, and define internal state for its next message to be handled.  This is said without defining specific opcodes and operands, though one could envision an abstract machine that supported that.