movement of electrons is only appears as movement of holes.But how we can say that met current is sum of hole current and electron current?
Yes, movment of electrons is only appears as movement of holes, but they are the not the free electrons, they are electrons in the neighbouring atoms. Since in a crystal lattice we have atoms arranged in periodic array an hole that is created in any atom by removal of an electron which becomes the free electron and can now constitute a current. And the hole can be filled up by hop or jump by an electron in adjacent atom, as this electron has moved the hole also has moved from first atom to next. And this continues and the hole can move through the array of atoms, and this is hole current and it's clearly distinct than the free electron current. So we have net current as sum of hole current and electon current.