In the 10th Doctor episode, "Journey's End," Davros is seemingly killed. However, he returns in the 12th Doctor two-parter "The Magician's Apprentice"/"The Witch's Familiar" with no explanation for his reappearance. How did he survive/escape the events of "Journey's End?"
Davros in the magician’s apprentice and the witch’s familiar had not experienced the events of the journey’s end
It’s not stated that Davros first experienced the events of the journey’s end before he experienced the events of the magician’s apprentice. The doctor is a time traveller. The tenth doctor might have met davros after the twelfth doctor.