Communication, or relation here means that two worlds (threads) access the same data and potentially hit the same cache line. E.g. a vCPU world and its networld for TX / RX and the vCPU at the destination (if the two VMs are on the same PG / VLAN / broadcastdomain). It could also be a vCPU and lets say the VM's pvscsi world with the IO world of the physical IO device. The VMs aren't sharing any memory, they reference the same data which is (outside of the VMs) in memory and more important, cache.