software threads vs hardware threads

A "hardware thread" is a physical CPU or core. So, a 4 core CPU can genuinely support 4 hardware threads at once - the CPU really is doing 4 things at the same time.

One hardware thread can run many software threads. In modern operating systems, this is often done by time-slicing - each thread gets a few milliseconds to execute before the OS schedules another thread to run on that CPU. Since the OS switches back and forth between the threads quickly, it appears as if one CPU is doing more than one thing at once, but in reality, a core is still running only one hardware thread, which switches between many software threads.

Modern JVMs map java threads directly to the native threads provided by the OS, so there is no inherent overhead introduced by java threads vs native threads. As to hardware threads, the OS tries to map threads to cores, if there are sufficient cores. So, if you have a java program that starts 4 threads, and have 4 or more cores, there's a good chance your 4 threads will run truly in parallel on 4 separate cores, if the cores are idle.

Software threads are threads of execution managed by the operating system.

Hardware threads are a feature of some processors that allow better utilisation of the processor under some circumstances. They may be exposed to/by the operating system as appearing to be additional cores ("hyperthreading").

In Java, the threads you create maintain the software thread abstraction, where the JVM is the "operating system". Whether the JVM then maps Java threads to OS threads is the JVM's business (but it almost certainly does). And then the OS will be using hardware threads if they are available.

Hardware threads can be thought of as the CPU cores, although each core can run multiple threads. Most of the CPUs mention how many threads can be run on each core (on linux, lscpu command gives this detail). These are the number of cores that can be used in parallel.

Software threads are abstraction to the hardware to make multi-processing possible. If you have multiple software threads but there are not multiple resources then these software threads are a way to run all tasks in parallel by allocating resources for limited time(or using some other strategy) so that it appears that all threads are running in parallel. These are managed by the operating system. Java thread is an abstraction at the JVM level.