<project>
   <!--
     ...
     Put your project's GAV and dependencies here
     ...
   -->
	<build>
		<plugins>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-antrun-plugin</artifactId>
				<version>1.6</version>
				<executions>
					<execution>
						<id>ini</id>
						<phase>initialize</phase>
						<goals>
							<goal>run</goal>
						</goals>
						<configuration>
							<!-- for plugin version < 1.5 <target> doesn't work - use <tasks> instead -->
							<target>
								<property environment="env" />
								<echoproperties prefix="env." />
							</target>
						</configuration>
					</execution>
				</executions>
			</plugin>
		</plugins>
	</build>
</project>

mvn-antrun-printenv

Prints names and values of all the environment variables from a context of maven build using maven-antrun-plugin and echoproperties maven task

Print all of the environment variables from a maven build

XML