A root partition is the isolated area in a Microsoft Hyper-V environment where the hypervisor runs. The root partition is the first one created; it starts the hypervisor and can access devices and memory directly.
In Hyper-V, there are three possible types of partitions: the root partition, the parent partition and the child partition. (In Microsoft Hyper-V version 1.0, however, the root and parent partitions are a single entity, so the terms are interchangeable in that context.) Each partition has its own memory and processor resources as well as policies on device use.
After the root partition starts the hypervisor, the parent partition requests creation of child partitions. The child partitions are where virtualized operating systems (Guest OS) and applications run. There can only be a single parent partition but the number of child partitions is only limited by the available resources.