For those of you who are so inclined to run Hyper-V as a nested VM on ESXi 5.5 (not sure why anyone would want to do such a thing), you should be aware that there is a new guestOS identifier that you can configure your VM to for the most optimal configuration. The main reasons you would want to use this configuration is that by default Windows Enlightenment is enabled and this will prevent Hyper-V from running as it will detect it is running inside of a VM. This configuration will disable Windows Enlightenment to allow you to run Hyper-V.
I noticed a new guestOS identifier called "windowsHyperVGuest" while browsing through the vSphere 5.5 API Reference guide, but when I checked the vSphere Web Client, I did not see this guestOS type as an available option. Perhaps this was not a supported guestOS, after all Nested Virtualization is not officially supported by VMware. In any case, you can still configure your VM by leveraging the vSphere API.
Here is a quick vSphere SDK for Perl script called changeGuestOSID.pl which allows you to reconfigure a VM with a valid guestOS identifier from the vSphere API Reference guide. You can of course easily do this using PowerCLI as well as any other language that can speak to the vSphere API.
Once updated, you should now see it reflected in both the vSphere Web/C# Client:
Note: I did not do extensive testing other than basic installation of latest Hyper-V Server and I do not believe you need any additional settings. If you wish to run nested 64-bit VMs, then you will need to enable VHV.