There are two common mistakes that people make when they setup a Hyper-V VM.
First problem is that they don't install the integration components before they start troubleshooting problems with the Hyper-V VM functionality. One thing the integration components do for the VM is to give the VM a network driver.
Second problem is that they assume that the network driver will be installed during the installation of the VM but that is incorrect you must install the integration components within the VM for the network driver support. If the VM needs to be installed through a PXE boot then you must install a Legacy Driver for this support before you install the OS.
Now I have ran into problems even with the Integration components being installed the Network driver installed by the integration components won't be recognized by the OS. What the fix for this issue is to use the Legacy Network Adapter.
To install the integration components
Before you start make sure you have installed the most current Service Pack and updates for the Guest OS. Make sure you restart the Guest OS after installing the updates.
1. Logon to the Guest VM with Administrative rights.
2. From the Hyper-V menu select the Action Menu and choose the Insert Integration
Services Setup Disk option within the menu.
Once you have done the two above steps the installation of the Integration Services should start automatically. After the installation you will need to restart the Guest OS.
Installing a Legacy Network Adapter.
(Legacy Adapters aren't supported in Windows Server 2003 64-bit or Windows XP 64-bit)
1. Make sure your Virtual Machine is shutdown.
2. Right-click the Virtual Machine in Hyper-v select Settings.
3. Select Add Hardware.
4. On the Add Hardware page select Legacy Adapter.
5. Then click Add.
Once you have finished the above steps make sure you see your Legacy Adapter under the installed Hardware within the VM's settings. If you have any other Network Adapters listed make sure you remove those.