Gavan has worked at the FTSE 100 company since 2001 and will leave his role at the end of January.
Tony Wray, chief executive of Severn Trent, said: ‘Peter has led us through a period of most challenging issues for Severn Trent, dealing openly with the media, regulators and investors to close those issues down and establish the credentials we now have for openness, operational performance and market leadership.'
Gavan has held senior comms posts in fellow FTSE 100 companies National Grid and Invensys, as well as the oil industry with TOTAL and Ireland's Viridian Group.
He was also previously director of issues and crisis management with Burson-Marsteller and was a political journalist and leader writer at Westminster for the Evening Standard.
Gavan said: ‘My work with Severn Trent, for long periods incredibly demanding, is now complete and it is with great eagerness and anticipation that I can look forward to whatever big challenges await in my next role.'
During his time with the firm Gavan handled reputation issues concerning the criminal and regulatory investigations into submission of false data by the company. He also managed external comms during the 2007 flooding of Mythe Water Treatment Works and the loss of drinking water for a fortnight to more than 140,000 households in Gloucestershire and Worcestershire.