Is it possible to upgrade an existing postgresql 9.1 production system to latest Postgres 10.0 version?
The main requirement is to get rid of downtime. Please help me out!
Zero downtime is only possible by standing up a hot-standby then failing over to it. Same-server upgrade you can do via pg_upgrade but it does involve downtime. There are lots of material and options online, including the docs, for setting up hot-standby replication.
To clarify, you need to use logical replication here since the WAL format is not usable across versions.
pg_upgrade is your simplest option if you can handle its downtime.