1. Create a snapshot with XenServer

Stop the database to make sure snapshot is consistent.

# service postgresql stop
 * Stopping PostgreSQL 9.3 database server
   ...done.

postgresql

xenserver# xe vm-snapshot new-name-label="Pre PG 9.4 install" vm=exampleVMname
26441bbe-9e47-5590-a93f-5b3cd2ddf9b7

2. Install the PGDG repo

# sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'

# curl https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -

# apt-get update

3. Install the PGDG postgresql upgrade

# aptitude install postgresql-9.4
The following NEW packages will be installed:
  postgresql-9.4 postgresql-client-9.4{a} postgresql-contrib-9.4{a}
0 packages upgraded, 3 newly installed, 0 to remove and 115 not upgraded.
Need to get 0 B/5,281 kB of archiv

4. Remove the new PG server empty “cluster”

# sudo -u postgres pg_dropcluster --stop 9.4 main

5. Perform an upgrade which does a binary copy of the existing database

# service postgresql stop

# pg_upgradecluster -v 9.4 9.3 main
Disabling connections to the old cluster during upgrade...
Restarting old cluster with restricted connections...
Creating new cluster 9.4/main ...
  config /etc/postgresql/9.4/main
  data   /var/lib/postgresql/9.4/main
  locale en_CA.UTF-8
  port   5433
Disabling connections to the new cluster during upgrade...
...
Success. Please check that the upgraded cluster works. If it does,
you can remove the old cluster with

  pg_dropcluster 9.3 main

6. Remove the old postgresql 9.3 (Ubuntu) packages

# apt-get remove postgresql postgresql.*-9.3

# apt-get autoremove

7. Proceed with the Ubuntu upgrade

# apt-get dist-upgrade

# do-release-upgrade

And restore the PGDG repo

# sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'

# apt update

# apt full-upgrade