[UPDATE] this cleared up the libxv1 problem:
# dpkg --install --force-overwrite /var/cache/apt/archives/libxv1_4.3.0.dfsg.1-1_i386.deb
[/UPDATE]

I got the same thing when I tried to update xawtv and that required libxv1 to be updated too. Here's a similar "apt-get -t unstable install libxv1" error tree:

root@localhost:~# apt-get -t unstable install libxv1
Reading Package Lists... Done
Building Dependency Tree... Done
The following NEW packages will be installed:
libxv1
0 upgraded, 1 newly installed, 0 to remove and 894 not upgraded.
1 not fully installed or removed.
Need to get 0B/138kB of archives.
After unpacking 299kB of additional disk space will be used.
(Reading database ... 120028 files and directories currently installed.)
Unpacking libxv1 (from .../libxv1_4.3.0.dfsg.1-1_i386.deb) ...
dpkg: error processing /var/cache/apt/archives/libxv1_4.3.0.dfsg.1-1_i386.deb (--unpack):
trying to overwrite `/usr/X11R6/lib/libXv.so.1.0', which is also in package xlibs
dpkg-deb: subprocess paste killed by signal (Broken pipe)
Errors were encountered while processing:
/var/cache/apt/archives/libxv1_4.3.0.dfsg.1-1_i386.deb
E: Sub-process /usr/bin/dpkg returned an error code (1)
root@localhost:~#

the file `/usr/X11R6/lib/libXv.so.1.0' was even removed and the same error showed up.....