its depend upon your solution, if its a farm level solution then this is called Infrastructure change and this does required IIS. You can control this IIS reset in your Solution’s code but to registered the features you need IIS reset.
“In deployments that use the timer service, the deployment creates a timer job. This timer job is picked up by the timer service on each Web server in the server farm. Initially, manifest and feature manifests are parsed to find assembly and _layouts files, which are copied to the appropriate locations. All other files contained within a feature directory are copied to the feature directory. After solution files are copied to the target computers, a configuration reset is scheduled for all front-end Web servers; the reset then deploys the files and restarts Microsoft Internet Information Services (IIS). Solution features are then registered, and schema and definition files are committed to the configuration store.”
here is good blog for deploy solution without downtime:
http://blog.ithinksharepoint.com/2012/07/16/deploying-sharepoint-wsp-solutions-without-downtime/
