Potassium is a group 1 element with one valence electron and iodine is a group 7 element with 7 electrons in the outermost shell.
To become stable , Potassium tends to donate its outermost electron and iodine tends to take in the electron given out by potassium to complete its outermost shell.
Therefore when potassium forms a compound with iodine one electron is transferred from potassium atom to iodine atom