My definition is:
Digital twins - the idea of recreating a physical infrastructure (think of IoT) as a digital model; it;s where the name Digital twin (i.e. replica comes from)
Knowledge graph - knowledge base which is graph oriented and typically focus more towards the instances rather than the schema and complex inference
How the two terms are related? Many of the companies implementing Digital Twins are using Knowledge Graphs to represent their models. On top of it they build eventing so once something happens in the real world, it should be propagated also to the digital representation.
