I know that I can add command line parameters to the grub2 command line by editing /etc/default/grub according to this answer

How do I add a boot parameter to grub2 in Ubuntu 10.10?

However, that would apply to ALL kernels would it not?

How do I apply the command line parameters to specific kernels? i.e. only xen.

I'm wanting to append something like:

xen-pciback.hide=(06:00.0)

I'm guessing I need to add it somewhere in the file:

/etc/grub.d/20_linux_xen

Which contains:

#! /bin/sh
set -e

# grub-mkconfig helper script.
# Copyright (C) 2006,2007,2008,2009,2010  Free Software Foundation, Inc.
#
# GRUB is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# GRUB is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with GRUB.  If not, see <http://www.gnu.org/licenses/>.

prefix=/usr
exec_prefix=${prefix}
bindir=${exec_prefix}/bin
libdir=${exec_prefix}/lib
. ${libdir}/grub/grub-mkconfig_lib

export TEXTDOMAIN=grub
export TEXTDOMAINDIR=${prefix}/share/locale

CLASS="--class gnu-linux --class gnu --class os --class xen"

if [ "x${GRUB_DISTRIBUTOR}" = "x" ] ; then
  OS=GNU/Linux
else
  OS="${GRUB_DISTRIBUTOR} GNU/Linux"
  CLASS="--class $(echo ${GRUB_DISTRIBUTOR} | tr '[A-Z]' '[a-z]' | cut -d' ' -f1) ${CLASS}"
fi

# loop-AES arranges things so that /dev/loop/X can be our root device, but
# the initrds that Linux uses don't like that.
case ${GRUB_DEVICE} in
  /dev/loop/*|/dev/loop[0-9])
    GRUB_DEVICE=`losetup ${GRUB_DEVICE} | sed -e "s/^[^(]*(\([^)]\+\)).*/\1/"`
    # We can't cope with devices loop-mounted from files here.
    case ${GRUB_DEVICE} in
      /dev/*) ;;
      *) exit 0 ;;
    esac
  ;;
esac

if [ "x${GRUB_DEVICE_UUID}" = "x" ] || [ "x${GRUB_DISABLE_LINUX_UUID}" = "xtrue" ] \
    || ! test -e "/dev/disk/by-uuid/${GRUB_DEVICE_UUID}" \
    || uses_abstraction "${GRUB_DEVICE}" lvm; then
  LINUX_ROOT_DEVICE=${GRUB_DEVICE}
else
  LINUX_ROOT_DEVICE=UUID=${GRUB_DEVICE_UUID}
fi

linux_entry ()
{
  os="$1"
  version="$2"
  xen_version="$3"
  recovery="$4"
  args="$5"
  xen_args="$6"
  if ${recovery} ; then
    title="$(gettext_quoted "%s, with Xen %s and Linux %s (recovery mode)")"
  else
    title="$(gettext_quoted "%s, with Xen %s and Linux %s")"
  fi
  printf "menuentry '${title}' ${CLASS} {\n" "${os}" "${xen_version}" "${version}"
  if ! ${recovery} ; then
      save_default_entry | sed -e "s/^/\t/"
  fi

  if [ -z "${prepare_boot_cache}" ]; then
    prepare_boot_cache="$(prepare_grub_to_access_device ${GRUB_DEVICE_BOOT} | sed -e "s/^/\t/")"
  fi
  printf '%s\n' "${prepare_boot_cache}"
  xmessage="$(gettext_printf "Loading Xen %s ..." ${xen_version})"
  lmessage="$(gettext_printf "Loading Linux %s ..." ${version})"
  cat << EOF
        echo    '$xmessage'
        multiboot       ${rel_xen_dirname}/${xen_basename} placeholder ${xen_args}
        echo    '$lmessage'
        module  ${rel_dirname}/${basename} placeholder root=${linux_root_device_thisversion} ro ${args}
EOF
  if test -n "${initrd}" ; then
    message="$(gettext_printf "Loading initial ramdisk ...")"
    cat << EOF
        echo    '$message'
        module  ${rel_dirname}/${initrd}
EOF
  fi
  cat << EOF
}
EOF
}

linux_list=`for i in /boot/vmlinu[xz]-* /vmlinu[xz]-* ; do
        basename=$(basename $i)
        version=$(echo $basename | sed -e "s,^[^0-9]*-,,g")
        if grub_file_is_not_garbage "$i" && grep -qx "CONFIG_XEN_DOM0=y" /boot/config-${version} 2> /dev/null ; then echo -n "$i " ; fi
      done`
xen_list=`for i in /boot/xen*; do
        if grub_file_is_not_garbage "$i" ; then echo -n "$i " ; fi
      done`
prepare_boot_cache=

while [ "x${xen_list}" != "x" ] ; do
    list="${linux_list}"
    current_xen=`version_find_latest $xen_list`
    xen_basename=`basename ${current_xen}`
    xen_dirname=`dirname ${current_xen}`
    rel_xen_dirname=`make_system_path_relative_to_its_root $xen_dirname`
    xen_version=`echo $xen_basename | sed -e "s,.gz$,,g;s,^xen-,,g"`
    echo "submenu \"Xen ${xen_version}\" {"
    while [ "x$list" != "x" ] ; do
        linux=`version_find_latest $list`
        echo "Found linux image: $linux" >&2
        basename=`basename $linux`
        dirname=`dirname $linux`
        rel_dirname=`make_system_path_relative_to_its_root $dirname`
        version=`echo $basename | sed -e "s,^[^0-9]*-,,g"`
        alt_version=`echo $version | sed -e "s,\.old$,,g"`
        linux_root_device_thisversion="${LINUX_ROOT_DEVICE}"

        initrd=
        for i in "initrd.img-${version}" "initrd-${version}.img" \
            "initrd-${version}" "initrd.img-${alt_version}" \
            "initrd-${alt_version}.img" "initrd-${alt_version}"; do
            if test -e "${dirname}/${i}" ; then
                initrd="$i"
                break
            fi
        done
        if test -n "${initrd}" ; then
            echo "Found initrd image: ${dirname}/${initrd}" >&2
        else
    # "UUID=" magic is parsed by initrds.  Since there's no initrd, it can't work here.
            linux_root_device_thisversion=${GRUB_DEVICE}
        fi

        linux_entry "${OS}" "${version}" "${xen_version}" false \
            "${GRUB_CMDLINE_LINUX} ${GRUB_CMDLINE_LINUX_DEFAULT}" "${GRUB_CMDLINE_XEN} ${GRUB_CMDLINE_XEN_DEFAULT}"
        if [ "x${GRUB_DISABLE_RECOVERY}" != "xtrue" ]; then
            linux_entry "${OS}" "${version}" "${xen_version}" true \
                "single ${GRUB_CMDLINE_LINUX}" "${GRUB_CMDLINE_XEN}"
        fi

        list=`echo $list | tr ' ' '\n' | grep -vx $linux | tr '\n' ' '`
    done
    echo "}"
    xen_list=`echo $xen_list | tr ' ' '\n' | grep -vx $current_xen | tr '\n' ' '`
done