Schema Spy unable to find postgresql driver

I think the problem is you didn't add the classpath to the driver. You have to add your classpath like this

java -jar schemaSpy_5.0.0.jar -t pgsql -host 10.100.71.21[:5432] -db mydb -s public -u username -p password -dp /home/panx/postgresql-8.0-312.jdbc3.jar -o output/

If it does not work you can download the source code of this project. In the project there is pgsql.properties file in dbTypes folder. You can change it with your classpath and I think this solves your problem.

I'd like to add a few points.

1. You would be using Schema Spy with Graphviz. Versions 2.31+ does not add an entry to Window's System path variable. So after you download and install Graphviz update the path variable to have path till Graphviz's bin directory. (C:\Program Files (x86)\Graphviz2.38\bin)
2. Alternative to downloading entire project source to change the classpath in the property file would be to update the jar using 7-zip.
3. Open the jar file using 7-zip navigate to net/sourceforge/schemaspy/dbTypes/ right click pgsql.properties say edit.
4. Change the following entry for driverPath to make it point to jdbc jar file driverPath=D:/Work/JavaProjects/tools/postgresql-9.3-1100.jdbc4.jar
5. Click on save and update. This should do the trick.

First you need to download the actual driver Here.
Then run with set path to Driver For example for Windows

java -jar schemaSpy.jar  -t pgsql -host localhost:5432 -db _dbname_ -s _schena_name_ -u postgres -p  -o D:\RVA\postgreSQL\  -dp "D:\Program Files (x86)\PostgreSQL\pgJDBC\postgresql-42.2.5.jar"