What is the difference between Kartik Poornima and Dev Deepavali?

They are the same event — Kartik Poornima IS Dev Deepavali. The name ‘Dev Deepavali’ (the Diwali of the Gods) is used specifically for the celebration at the ghats of Varanasi, where the festival is observed with extraordinary grandeur. All across Bharat, this full moon is called Kartik Poornima and is celebrated with Deep Daan and holy baths.