K+ IS ALSO POSITIVELY CHARGED THEN HOW NERVE MEMBRANE BECOMES NEGATIVELY CHARGED WHEN K+ ENTERS INSIDE?

Axonal membrane is more permeable to K ⁺ means K⁺ can cross the cell membrane and come in neuron and nearly impermeable to Na ⁺  means Na⁺ can not cross cell membrane to come inside the neuron. 

It is impermeable to negatively charged proteins present inside the axoplasm means these proteins can not cross the membrane and go out. Thus, it does not let them go out.

As a result, the axoplasm has more concentration of K ⁺ and negatively charged proteins, and less concentration of Na ⁺ . This creates a concentration gradient means there is more positive charge present on the outside of the membrane of neuron as compare to inside. Thus the inside of membrane is taken to be negative with respect to outside.