Though Preventive Action term is often used with Quality Management Systems, but it is vital and used widely in all knowledge areas esp. Project Integration, HR, Quality & Risk.
Some processes recommend preventive actions as output and few other processes takes the approved preventive actions as input to bring the project into compliance with project management plans.
Preventive Actions are documented direction to perform an activity that can reduce the probability of negative consequences associated with project risks. 
Preventive actions are recommended in anticipation of possible problems and they are generally output from monitoring and controlling process group. Preventive actions may also include contingent actions taken to reduce the seriousness of a future problem if it should occur. Depends on the nature of responses to preventive actions, sometimes, they may results in a change request.
A simple example of Preventive action would be providing first aid kits in each block of organization and posting list of emergency phone numbers inside lifts.
Project manager and project management team along with all stakeholders need to review recommended preventive actions and debate on the effectiveness and implementation procedures before they are submitted for approval. Only approved preventive actions are later implemented.
Preventive action is a proactive process to identify opportunities for improvement rather than a simple reaction to identified problems. Preventive action include investigation, action, review, and further action if so required and follow Deming’s PDCA cycle.
In Quality management, Recommendation of new preventive actions is done in Quality control process. Preventive action involves action taken to prevent a condition that may exceed established parameters in a manufacturing or development process, which may have been indicated through a QC measurement.
In Quality Assurance process, quality audits confirm the implementation of approved change request, defect repairs, corrective actions, and preventive actions and it may recommends corrective actions.
Taking preventive actions in HR issues reduces the probability and impact of resource problems before they occur. Examples to preventive actions are: training resources beforehand to reduce problems during resource crunch, clarification on roles and responsibilities, project progress and deadlines can increase the team buy-in and reduce problems when they are asked to put extra effort to meet project deadlines.
As mentioned in my earlier post on stakeholder management & analysis, by anticipating stakeholders’ reaction to the project, taking preventive actions can be taken to win their support or minimize potential negative impacts.
Root cause analysis combined with corrective action to help understand the cause of the deviation and potentially prevent recurrence of a similar problem.
. PMBOK® Guide – 3rd edition