Memory Mapped I/O :

Memory mapped IO uses one address space for memory and input and output devices. In other words, some addresses are assigned to memory while others are assigned to store the addresses of IO devices. There is one set of read and write instruction lines. The same set of instructions work for both memory and IO operations. Therefore, the instructions used to manipulate memory can be used for IO devices too.  Hence, it can lessen the addressing capability of memory because some are occupied by the IO. 

Instruction-based I/O :

I/O instruction One of a class of instructions that describes the operations concerned with input and output.

Difference  Between Memory mapped I/O and Instruction- based I/O is 

Memory mapped IO is a method to perform input/output (I/O) operations between the central processing unit (CPU) and peripheral devices in a computer that uses one address space for memory and IO devices. IO mapped IO is a method to perform input/output (I/O) operations between the central processing unit (CPU) and peripheral devices in a computer that uses two separate address spaces for memory and IO devices.

Thus, this definition explains the basis of the difference between memory mapped IO and IO mapped IO.