This is on Ubuntu 12.04LTS using KVM. The system has a graphics card which is visible to the hypervisor's kernel shown as follows:

# virsh nodedev-dumpxml pci_0000_21_00_0
<device>
  <name>pci_0000_21_00_0</name>
  <parent>pci_0000_20_02_0</parent>
  <driver>
    <name>nvidia</name>
  </driver>
  <capability type='pci'>
    <domain>0</domain>
    <bus>33</bus>
    <slot>0</slot>
    <function>0</function>
    <product id='0x0a65'>GT218 [GeForce 210]</product>
    <vendor id='0x10de'>NVIDIA Corporation</vendor>
    <capability type='virt_functions'>
    </capability>
  </capability>
</device>

There is also a PCI Function #1 which is the audio portion of the card for which I omitted the dumpxml.

In my domain XML, I have the following:

<hostdev mode='subsystem' type='pci' managed='yes'>
  <source>
    <address domain='0x0000' bus='0x21' slot='0x00' function='0x0'/>
  </source>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0' multifunction='on'/>
</hostdev>
<hostdev mode='subsystem' type='pci' managed='yes'>
  <source>
    <address domain='0x0000' bus='0x21' slot='0x00' function='0x1'/>
  </source>
  <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x1'/>
</hostdev>

Without these hostdev tags my guest VM boots up just fine (it is also an Ubuntu 12.04LTS kernel). When I try to start the VM with them I get:

# virsh nodedev-detach pci_0000_21_00_0
Device pci_0000_21_00_0 detached

# virsh nodedev-detach pci_0000_21_00_1
Device pci_0000_21_00_1 detached

# virsh start testv
error: Failed to start domain testv
error: Unable to read from monitor: Connection reset by peer

Can anyone say what is going wrong?

UPDATE: Being somewhat new to KVM I didn't realize there was a log file available. It says this:

Failed to assign device "hostdev0" : Device or resource busy
*** The driver 'pci-stub' is occupying your device 0000:21:00.0.
***
*** You can try the following commands to free it:
***
*** $ echo "10de 0a65" > /sys/bus/pci/drivers/pci-stub/new_id
*** $ echo "0000:21:00.0" > /sys/bus/pci/drivers/pci-stub/unbind
*** $ echo "0000:21:00.0" > /sys/bus/pci/drivers/pci-stub/bind
*** $ echo "10de 0a65" > /sys/bus/pci/drivers/pci-stub/remove_id
***
kvm: -device pci-assign,host=21:00.0,id=hostdev0,configfd=30,bus=pci.0,multifunction=on,addr=0x4: Device 'pci-assign' could not be initialized
2014-12-17 19:15:52.368+0000: shutting down

So apparently the nodedev-detatch command leaves a stub driver active? Are the suggested commands to be run in the hypervisor or the VM?