Zynq device allows the development of application involving usage of both Processing System(PS) as well as Programmable Logic. Both Processing System and Programmable Logic have Independent circuitry hence can be independently shut down depending on the subsystem usability and power requirements.

All the Zynq devices in general consists of dual-core ARM Cortex A9 Processor as Processing System. Zynq PS is a Hard processor so it is a dedicated silicon element residing on FPGA Chip. PS is not reconfigurable as opposed to Microblaze soft processor. The Soft processor is similar to IP that can implement different IP blocks if unused but the same cannot be done with Hard Processor.  Zynq PS outperforms in performance (at least 6 - 7 times fast) as compared to Microblaze hence whenever we as an engineer have the constraint of performance we choose Zynq by default.

ARM provides to its user which provide the flexibility to trim the architecture So when Xilinx receive ARM copy they perform some optimization to glue it easily with existing FPGA architecture along with good performance. So if you wish to best utilize ARM core prefer Xilinx Manual instead of ARM official document.

Xilinx SDK acts as an environment for the ARM processor.