Upstream fix:

https://github.com/libguestfs/libguestfs/commit/be1c6965006dec258b915c9561ac66c8cb36dfe6

This will appear in libguestfs >= 1.27.40.

I can also reproduce this bug issue with new package version:
virt-v2v-1.27.41-1.1.el7.x86_64
libguestfs-1.27.41-1.1.el7.x86_64

1.Prepare a guest with no space left.
# df -h
Filesystem                    Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup-lv_root  6.5G  6.5G     0 100% /
tmpfs                         939M   76K  939M   1% /dev/shm
/dev/sda1                     485M   35M  426M   8% /boot

2.Use virt-v2v to convert the guest.
# virt-v2v -o rhev -os 10.66.90.115:/vol/v2v_auto/auto_export --network rhevm demo
[   0.0] Opening the source -i libvirt demo
[   0.0] Creating an overlay to protect the source from being modified
[   0.0] Opening the overlay
[   2.0] Initializing the target -o rhev -os 10.66.90.115:/vol/v2v_auto/auto_export
[   3.0] Inspecting the overlay
[  11.0] Checking for sufficient free disk space in the guest
[  11.0] Converting Red Hat Enterprise Linux Server release 6.5 (Santiago) to run on KVM
virt-v2v: error: libguestfs error: command: cp: writing 
`/tmp/initramfs.ClQe2q/lib/kbd/unimaps/cp437.uni': No space left on device
E: Failed to install /lib/kbd/unimaps/cp437.uni

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]

Since the error has not been improved, assigned back, and please check whether the patch in Comment 1 has been included by libguestfs package, thanks.

(In reply to zhoujunqin from comment #3)
> I can also reproduce this bug issue with new package version:
> virt-v2v-1.27.41-1.1.el7.x86_64
> libguestfs-1.27.41-1.1.el7.x86_64
> 
> 1.Prepare a guest with no space left.
> # df -h
> Filesystem                    Size  Used Avail Use% Mounted on
> /dev/mapper/VolGroup-lv_root  6.5G  6.5G     0 100% /
> tmpfs                         939M   76K  939M   1% /dev/shm
> /dev/sda1                     485M   35M  426M   8% /boot

df isn't a very accurate way to display the actual free space in
a filesystem.  It shows free space available to non-root users,
and the -h option obscures things further.

Use:

$ stat -f /

instead.  I'm betting there is free space on this filesystem, but
df is just not showing it to you.

I tried as you suggestion on guest.
By root user:
# stat -f /
  File: "/"
    ID: dd12bb426bbe0a3a Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 1687234    Free: 5401       Available: 0
Inodes: Total: 429088     Free: 333905

By normal user:
$  stat -f /
  File: "/"
    ID: dd12bb426bbe0a3a Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 1687234    Free: 5352       Available: 0
Inodes: Total: 429088     Free: 333878

do you think it's ok to prove the guest with no space left.

(In reply to zhoujunqin from comment #5)
> I tried as you suggestion on guest.
> By root user:
> # stat -f /
>   File: "/"
>     ID: dd12bb426bbe0a3a Namelen: 255     Type: ext2/ext3
> Block size: 4096       Fundamental block size: 4096
> Blocks: Total: 1687234    Free: 5401       Available: 0

You're not quite triggering the error message.

The disk itself has 5401 free blocks.  Because the block size
is 4096 bytes, that means you have 22122496 bytes free.

The "Available: 0" in this case means that non-root users are
unable to use this space, because Linux filesystems reserve a
percentage of their space for the root user.  (Use 'tune2fs -l'
to find out what percentage).  Non-root users are irrelevant
because all conversion operations are done ignoring this
reserved space.

virt-v2v checks to see if you have at least 20000000 bytes
free on the root partition, which in this case you do
(20000000 < 22122496).  So the error is not printed.  But
the margin is so small that if you created even a 3 MB file on
the root filesystem then you should get the error.

Thanks for your help, Richard W.M. Jones.
I tried again with package:
virt-v2v-1.27.41-1.1.el7.x86_64
libguestfs-1.27.41-1.1.el7.x86_64

1. Prepare a guest with no space( Login guest and check)
# stat -f /
  File: "/"
    ID: dd12bb426bbe0a3a Namelen: 255     Type: ext2/ext3
Block size: 4096       Fundamental block size: 4096
Blocks: Total: 1687234    Free: 4096       Available: 0
Inodes: Total: 429088     Free: 333880

2. Use virt-v2v to convert the guest.
# virt-v2v -o rhev -os 10.66.90.115:/vol/v2v_auto/auto_export --network rhevm demo
[   0.0] Opening the source -i libvirt demo
[   0.0] Creating an overlay to protect the source from being modified
[   0.0] Opening the overlay
[   2.0] Initializing the target -o rhev -os 10.66.90.115:/vol/v2v_auto/auto_export
[   2.0] Inspecting the overlay
[  11.0] Checking for sufficient free disk space in the guest
virt-v2v: error: not enough free space for conversion on filesystem '/'.  
16887808 bytes free < 20000000 bytes needed

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]

Can get the friendly error message, so move from ASSIGNED to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-0303.html