This was taken from hp's website hope it helps we gets asked this question a lot what is the difference between pcl5 and pcl6.
 

 

Hewlett-Packard developed the PCL printer
language to standardize access to printer features.
What is PCL?
The PCL (Printer Command Language) language provides the highest level of communication between the system and the printer. It is independent of the host system, device drivers, I/O interface, and network communications. Its purpose is to bring all HP printers together under a common control structure. The feature compatibility provided by the PCL language protects the user's investment in applications and driver software and provides a vehicle for exploiting new features, capabilities, and technologies.
What is PS?
THE POSTSCRIPT LANGUAGE is a simple interpretive programming language with powerful graphics capabilities. Its primary application is to describe the appearance of text, graphical shapes, and sampled images on printed or displayed pages, according to the Adobe imaging model. A program in this language can communicate a description of a document from a composition system to a printing system or control the appearance of text and graphics on a display. The description is high-level and device-independent.
Printing from the computer - Common Print Path
When printing from a Windows application such as Microsoft PowerPoint, the Windows application outputs the print job as an EMF file. Please refer to figure below. The EMF (Enhanced Meta File) format is an image description format defined by Microsoft Windows. The printer driver on the server converts the EMF file to the RAW data format appropriate for the target printer. In the Windows' printing system, RAW data is defined, as the data that is sent to the printer in a printer language the printer understands, such as PCL5/6 or PCL3Plus.
The difference in the languages is in the resultant RAW data files. The size of the RAW data 'file' is important because it represents the amount of I/O bandwidth that must be used to get the data to the printer. Please refer to Figure 2.
Figure 1: Data flow
The advancement in the PCL versions is to reduce the size of the RAW file to improve performance. Print quality and performance is the key to customer satisfaction for the imaging solution. This document offers a comparison of the PCL versions as well as highlights the current advancements to the PCL solution - the PCL 6 solution.
Differences between PCL 5/6 and PCL 3
  PCL 5/ 6 PCL 3
1 Prints complex graphical images in standard memory To print out some graphical data, the printer needs a faster image processing printer language like PCL3GUI or High Performance Architecture.
2 Object-based language used to describe images. Many objects such as squares, rectangles, lines, arcs, circles and etc can be defined by object-based language. Benefit: For these objects, the image can be described using less data than the corresponding pixel description used in PCL3GUI. Pixel-based printer language that primarily describes images by pixel. For the same objects like square, rectangles, etc, more data is required. Although bitmaps and other complicated picture type objects use object based language, they are still described in pixels because of their complexity.
3 PCL 5/6 supports downloadable fonts as well as scalable fonts. It also has better capability to rasterize the image compared to the GL2 driver. Does not support downloadable or scalable fonts
4 Conversion time from EMF to the PCL5/6 object data is less. This reduces the CPU time required. Users have all of the resources of the computer available to them more quickly after starting a print job. For most EMF object and images, it takes more Central Processing Unit (CPU) bandwidth to convert the EMF to the PCL3GUI pixel data.
5 PCL5/6 printers need more printer memory or at times printer hard disk to rasterize the fonts/images in printer instead of computer system. The PCL3GUI driver, in general, will use more memory on the computer than the PCL5/6 drivers when creating the RAW data files. PCL3GUI printers appear as a less expensive overall printing solution as the initial purchase price is usually less compared to the PCL5/6 printers. This is because they require less memory RAM and less processing power on each printer.
6 In a network environment, a print server can efficiently control more PCL5/6 printers than PCL3GUI printers. Generally, PCL5/6 printers with higher print speed are for network-enabled printers When using a client/server use model where print jobs are sent from the client to the server and formatted on the server, the more the users share a fixed bunch of personal printers networked together using the same server, the slower the performance.
PCL 6 features
PCL 6 is the latest evolution of the Hewlett-Packard PCL language and related technologies. PCL 6 has three key components.
  • The first component, referred to as PCL 6 Enhanced has been optimized for printing from graphical user interfaces (GUIs) like Microsoft Windows and OS/2.
  • The second component referred to as PCL 6 Standard provides complete backward compatibility with past HP LaserJet printers.
  • The third component referred to as Font Synthesis provides scalable fonts
The PCL6 driver provides enhanced WYSIWYG (What You See Is What You Get) and enhanced performance with application support over the Standard driver. PCL XL is a new page description language by HP that is part of PCL6 and is closer to GDI, which many applications use. Less translation takes place by the driver, which means increased WYSIWYG capabilities and better performance with applications that support escapes sequences implemented by the enhanced driver.
The number of commands and data required to describe a graphics page is reduced. This helps the user to return to the application faster and print complex graphic pages in standard memory. By matching the graphics commands in the printer to the graphics interface of the GUIs, users experience true WYSIWYG for all pages.
An object-oriented printer language
PCL 6's new architecture uses object-oriented commands to communicate with the printer. Object-oriented commands are small, succinct bits of data that are used to tell a printer what to print on a page. Using object-oriented commands instead of long strings of code sequences dramatically improves performance for graphics printing, offers faster return to applications and reduces network traffic.
Improved performance
PCL 6 commands were designed by HP to closely match Microsoft Windows graphical direct interface (GDI) commands. As a result, users get improved performance and better print quality. For example, users will regain control of their PC and application sooner because PCL 6 allows the printer driver to translate GDI to PCL 6 faster than before. Network throughput will improve because PCL 6 uses smaller, more compact commands than previous versions used. Complex graphics will print faster because PCL 6 is specifically designed for graphics while gray-scale images, which are processed as objects, appear smooth and even, offering superior print quality for graphics and scanned images. Design consistencies between PCL 6 and GDI also yield better WYSIWYG printing.
Although PCL6 generates smaller file sizes in certain network configurations, the speed of today's host processors and networks may offset the performance benefits of having smaller file sizes.
In which situations will customers prefer PCL5
PCL6 is not popular with ISVs (Independent Solution Vendors) who are reluctant to move away from the PCL5 solutions that they have already invested in. Major applications like SAP generate their own PCL5 data streams. ISVs, SAP, and customers are unlikely to switch to PCL6.