In some case if you get any error while trying to migrate some of the virtual machines running on vmware or xen using virt-v2v tools and if you didn't get any error output on the command line then you can increase the virt-v2v debug level verbosity by following the below steps to narrow down and fix the issue .
Check "netcat" is installed on the machine that is executing virt-v2v from?
Log in to the server console ( where v2v is installed ) a root and execute the below command to verify netcat is installed or not on the host .
# which nc
# rpm -qf /usr/bin/nc
If you didn't find any then use the below command to install it .
yum install nc
Please enable debugging by running following commands to get debug logs on the same terminal console where you executed the above commands .
# export LIBGUESTFS_TRACE=1 # export LIBGUESTFS_DEBUG=1
Now try the earlier virt-v2v command on the same terminal console and check the console logs for detailed error output