100% CPU in the guest could correspond to [25%] CPU in the host.
It depends on the number of physical CPUs in the host and the
number of virtual CPUs in the guest.

Please give some more details of the host, such as number of
physical CPUs.  Also please give the guest libvirt XML.

I have 4 cpus in the host, and 1 vcpu in guest.

As you said, I'll try to increase the vcpu num to 4 for the test.

# virsh dumpxml xp
<domain type='kvm' id='17'>
  <name>xp</name>
  <uuid>18585065-c348-1c4f-46b9-fdfee279543b</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64' machine='rhel6.3.0'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/winXP-32-virtio.qcow2'/>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <controller type='usb' index='0'>
      <alias name='usb0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='virtio-serial' index='0'>
      <alias name='virtio-serial0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:3a:5f:6c'/>
      <source network='default'/>
      <target dev='vnet0'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/4'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/4'>
      <source path='/dev/pts/4'/>
      <target type='serial' port='0'/>
      <alias name='serial0'/>
    </console>
    <channel type='spicevmc'>
      <target type='virtio' name='com.redhat.spice.0'/>
      <alias name='channel0'/>
      <address type='virtio-serial' controller='0' bus='0' port='1'/>
    </channel>
    <input type='tablet' bus='usb'>
      <alias name='input0'/>
    </input>
    <input type='mouse' bus='ps2'/>
    <graphics type='spice' port='5900' autoport='yes'/>
    <sound model='ich6'>
      <alias name='sound0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </sound>
    <video>
      <model type='qxl' vram='65536' heads='1'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <video>
      <model type='qxl' vram='65536' heads='1'/>
      <alias name='video1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </video>
    <video>
      <model type='qxl' vram='65536' heads='1'/>
      <alias name='video2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </memballoon>
  </devices>
  <seclabel type='dynamic' model='selinux' relabel='yes'>
    <label>system_u:system_r:svirt_t:s0:c605,c770</label>
    <imagelabel>system_u:object_r:svirt_image_t:s0:c605,c770</imagelabel>
  </seclabel>
</domain>

Not clear what you're expecting to see.

If this is a real bug, clone it for Fedora and provide a
clear test case.

Also, it might be a problem in libvirt (which collects the
stats) rather than virt-top (which just displays them).

Close it as NOTABUG according to comment 1, and it's the expected result for virt-top.