At customer location we're using multiple Hyper-V hosts, managed by a System Center Virtual Machine Manager (SCVMM) server. On multiple Virtual Machines (VM's) we like to deploy a server operating system with ConfigMgr. Trick is, when booting Hyper-V VM's from a PXE server, nothing seems to happen. Within Hyper-V Manager > BIOS config, the "Legacy Network adapter" is on top. Just have a look at the comment: "Use a legacy network adapter to perform a network-based installation of the guest operating system."
Because "Legacy Network adapter" is displayed here, we're thinking that the right network adapter is installed in the VM. Unfortunately this is not the case. Within VM properties in SCVMM there's a choice to use a network adapter or legacy network adapter. By default the network adapter is installed. With this network adapter no PXE boot can be done. Just delete this network adapter or choose "not connected" and add a new legacy network adapter.
After that PXE boot is working immediately. Just remember the following: Hyper-V supports booting a VM from the network using the F12 option. Trick is that you must use the legacy network adapter. The regular network adapter is synthetic and therefore not available at boot time. You have to remove the existing network adapter and add a new legacy network adapter. With that you're done!